It rarely has any further detail, explanation or depth to the opinion either. It's like some sort of popular echo chamber rallying descriptor that doesn't actually mean or say much in the grand scheme of things. I can find all manner of things fun in games, including glitches, broken mechanics, weird exploits and so on, but that wouldn't necessarily mean those things constitute good or competent gameplay design.
Regarding the whole fun thing, there are levels to these things. Even bad games can have fun elements, what's important is how well a game can sustain fun gameplay or quality design, and from what I can gather in the reviews, most don't feel the core gameplay loop is enough to sustain consistency of engagement or fun over a prolonged period of time. The general gist is that it is so shallow, hollow and repetitive, that the fun factor is detrimentally impacted, and the actual core gameplay design itself, lacklustre.
And that's where the whole breaking new ground or offering more thing comes into play. This isn't 2007, this is over a decade later. The expectation and level of quality have moved on, so more of the same isn't necessarily enough today even if it was over a decade ago. Not only can overt familiarity sometimes be a negative thing, but it can be even more of a negative if coupled with a lack of modernisation in terms of levels of polish, animation quality etc, and if we're being honest, I'm sure most could agree that Crackdown 3 does look fairly under par in those respects. It not only looks like a previous gen game in terms of gameplay and/or design but also in terms of animations, movement etc too, and I'd imagine that might be working against it.
In truth I'm actually baffled when I read people say they just wanted more Crackdown. Literally just more of the same with little improvement, innovation or modernisation. That sort of outlook is just so inhibiting and myopic in terms of lowered expectations and standards from studios and publishers.