I can vouch for this. I'm currently stuck in the beach world on day 28 (lose fewer than five plants) and the zombies they throw at you are ridiculous: surf zombies, cthulu zombies, fisherman zombies, and the giant crab zombie together in a single match is bonkers.
I can't wait to move on to the next world.
Beach World was definitely the hardest from my experience too. Which was weird since it's pretty much the middle of the game.
-Ice World was fine, I don't remember anything too bad about it other than the gimmick of stopping your plants from freezing.
-Aztec World was pretty easy from what I remember because you get a lot of sun by planting on special tiles, there are some annoying zombies, but they're manageable.
-80's World got tough because of some of the new zombies.
-Dinosaur World is tough, but actually pretty fun. The dinosaur gimmick works surprisingly well since you get a new plant to manage them and you also get some GREAT new plants that help you in all the areas (a new Wall Nut that is tougher and can be planted quicker, and a new potato mine that arms faster and does an AOE explosion like the cherry bomb).
-Modern day is just a mix of all the zombies from the other areas. It has some REALLY tough levels, some that require almost PERFECT reflexes in addition to really specific plant selection, but I was able to do most of them without too much trouble.