Double post because instead of talking about the mobile structure I wanted to talk about the revamped gameplay and mechanics!
So like I said before, they got rid of Sunflower which greatly speeds up the start of each fight. You might think that gets rid of strategy, but not really. It just keeps you from having to sit there waiting for some sunflowers to grow and saves you a lot of tapping to collect sun. Every fight has two phases, and between them you can use tacos to buff your plants and move the plants around. I really like this change as it gives you a bit of leeway early on to get sloppy as long as you can bring it back by the second phase. For example, if a zombie gets past my initial line of defense and I have to place a Wallnut near the back to stop him, I don't have to get rid of that Wallnut afterwards since I can just move it to the front line between phases.
Tacos are also a new gimmick that lets you upgrade a plant in some fashion. The Peashooter will increase its fire rate, the Wallnut will get armor, the Snapdragon will be able to hit flying enemies, etc. This happens between phases and adds some more strategy as who you upgrade can save your bacon in the second phase. I like to put two Snapdragons in the second and fourth spots behind armored Wallnuts, ensuring they hit every lane and also stop flying attackers. The trick is making sure I have two Snapdragons by the end of the first phase (or enough sun to plant one), because if I don't have them then, I can't upgrade them later.
Plant food is also back from PvZ2 and works roughly the same way, it's a super move that slowly fills up over the course of the battle that you can give to any plant. I recall in the second game I wasn't a big fan of the mechanic as it felt more like a cheat, which wasn't helped by the fact that you could straight up buy some mid-battle if you were doing poorly. You can't do that here, and you definitely need to be smart with how they're used because the zombies in this game don't fuck around. I also just unlocked the ability to use one cherry bomb per battle as a Hail Mary if things go sideways and you have no other option. That one does feel more like a cheat, but with the increased difficulty I ain't complaining.
This game also gives you a list of every zombie that can appear before every battle as well as their specific traits and what plants are effective against them, a very nice addition. Overall the difficulty seems well balanced, the zombie breakouts are pretty casual and not too tough which lets you play more with the scoring aspect and makes 2x speed viable to play with, and the tower fights start getting hectic quick for some real challenge and mercifully do not test you on your score.