I really enjoyed the first Pothead Zombies, and this one is very good too. 5 stars for sure. The difficulty is quite balanced, I only got a bit stuck on level 10 with all those buses, but eventually got over it.
I don't think the main difficulty of the game is lack of ammo. You have to balance your use of weapons, at some points during each wave you can change to a worse weapon and save ammo for the really tough moments.
The only thing that made me a bit frustrated sometimes was the skills of the coop player. I like the idea of the coop being better with some weapons than with others, because you can use it to build both characters as complementary. But even in weapons where the coop starts with 4/9 skill level, the guy is really bad (meaning it shoots super slowly) and it takes a lot of time to learn. Eventually, by the last waves you get your coop to be very good at one weapon and sort of OK with another and you just use it all the time, which limits choices in weapon selection.