A review is essentially just an individual's impressions with a game, their opinion. If a game is so awful they can't finish it, well, that's as informative in of itself that they had that experience as anything else IMO.
As for the "okay, but what if there were two reviews, one of them from someone who finished the game, the other didn't, who would you listen to" hypothetical that keeps popping up in this thread and other variants of it, that one is pretty easy really.
In that situation, since one of them finished it, the other didn't, it's highly likely they have very different tastes in games/different tolerances for different things. Thus, who I would be more likely to listen to in that situation would be the one of them who's tastes are most similar to my own, based on their respective previous reviews. Like reviews in general are supposed to work. Pretty straightforward.
Though I have to say the thing that annoys me the most in this thread is seeing variants of "it's their job" over and over again. Because it's just rude and unnecessary in multiple ways.
Because like... It's their job, not mine. Who am I to say what their job is or isn't? I think they would know better than me. And indeed if their boss says to have to finish it, fine. But if their boss is cool with them not finishing it, and is just fine with them not finishing a game after giving it their best shot because that's how bad the game is and they have to move on to playing something else and so writing a review based on the experience they did have, if their boss is cool with that... Well, clearly finishing the game is NOT necessarily part of their job after all in that situation. And they would know much better than me.
And that's what I'm getting at there: that yknow, just in general, like with any job in a given industry, rules can be completely 100% at different businesses within the same industry, that different bosses and companies can be completely different even if the basic job itself is the same, and that's something we be probably all experienced to different degrees. And the point being that if their boss is cool with it, who am I to say any different?
That's between them and their boss, their place of employment. If their boss is indeed cool with it, who am I, a complete nobody, to say any different? They clearly know better and in that situation it clearly isn't part of their job. That's just not something you can throw such blanket statements onto, and it's so condescending to tell someone else what the requirements for their job ate or aren't when you aren't the one who makes those decisions or knows anything about that, at all, so the "it's their job" argument just really, really gets on my nerves for all of those kind of reasons and more.
But I digress. Something else to consider is just how arbitrary the goal of getting to the end of a game really is. Like, it doesn't seem that way at first obviously. It seems to make sense. It's about "experiencing everything a game can offer" and how experiences can sometimes change right at the end. But everything has to offer... Usually, especially these days, the ending itself isn't all a game has to offer. And indeed if an opinion can change getting to the end, it can certainly change from getting 100% completion as well. But that's not a very reasonable expectation, obviously, for reviewers to not only beat s game, but also 100% it.
Now obviously a follow-up to that is 'can they just play 5-minutes then" but obviously to begin with that's just a reductive slippery slope and not something I want to entertain that much, and that aside, just like with finishing a game, that would be between them and their employer and if said employer is fine with it, it's not really for me to question that.
The point being though, the ending is just as arbitrary as anything else and experiences and opinions could potentially change at any point, at any time playing a game and there's nothing special about the ending itself. This, if a game is so terrible that a reviewer can't compell themselves to finish, not only is that informative in of itself of their experience of the game, I can't personally nake a good argument based around compelling them to continue base around the ending being able to change their mind or whatever because yeah, it could.... But it just as easily couldn't. And the same is true of each point on the game, not just the ending. And none of that changes that if the game was indeed frustrating or otherwise terrible enough to even make them consider such a thing, that's still nonetheless informative in of itself.
Plus, there are very interesting cases in points of what even constitutes an ending of a game anyway. Like, the easy example here is say stuff like many MMOs which often by their very nature don't really end per se. And then even better, games that don't have any real story or anything of the sort, like multiplayer-only/PVP-only based games, y'know, stuff like your Overwatches and CS:GO and all that. An ending isn't always available anyway. What do you do with games like that? How many hours are you "required" to play?
It's all arbitrary. As long as the reviewer has given it a solid try, as long as they've given it their best effort and they indeed believe they've experienced enough to have solidified their thoughts on the game and put them into the form of a review, that's what counts, and generally speaking, key word being generally speaking here before someone tries to "what about..." this, generally speaking the reviewer is more likely than me to know what "enough" is to form their own personal opinion on a game.
Then, there are even more fun examples of how arbitrary it all is. Like, imagine if Castlevania: Symphony of the Night was being released for the first time. Imagine if a reviewer got to the "bad" ending of the game and indeed thought they were finished. Should they be able to finish?
Because indeed, just by the point that you're able to fight Belmont you'll have the majority of the map filled in. You would have gotten like your items and transformations and stuff that Death took from you at the beginning of the game back.
And yeah, it's strange that Belmont is suddenly evil. But the game goes out of it's way to give him a motive and explanation for that. That being a vampire Hunter only brings temporary fame but eventually doomed them to irrelevance, but by becoming a Lord of darkness himself, he can become immortal, and just having become that obsessed with his legacy. And obviously heroes going bad as a huge plot twist is hardly unheard of to put it mildly and it's not like Alucard himself even dwells on it that much in the bad ending and is basically all "well, I guess my work is done," so, yeah.
Now obviously is ones a larger Castlevania fan one can tell that certain things are still off, like say how you don't actually fight Death at all if you get that ending and that's kinda weird, but nonetheless, especially if it's one's first Castlevania game, the point being I'd that one could easily be forgiven if they thought that wasn't just an ending but THE ending of the game
So what to do in situations like that? Where they genuinely get an ending, believe it's the ending, and have no particular reason to think there's more? Personally, can't think of any reason, in that situation that they shouldn't be able to write a review at that point, just because they don't have the knowledge that theres a different way of playing the game that would lead to different results.
And if one feels otherwise with SOTN in particular this becomes more clear with other games with multiple endings like say Undertale. What's required to review Undertale? Is just a few neutral ending variations enough? Do they have to get True Pacifist? What about Genocide? Personally speaking, especially for a game like Undertale where it's all about the player playing their own way, anything is fine with me and it doesn't really make that much sense to put those kind of restrictions on reviews for it, but that's me.
The point being to show that even in critically acclaimed and well-known games how even something straight forward like "beat the game" isn't necessarily as simple as it appears.
So yeah, the long and short of it is for me, there are no universal answers to a question like that, at least that's how I feel.
