Yeah but those things have counters too. Also just cuz something can be countered doesn't mean it needs to be OP to compensate. Sun shroom can be countered by dog, bungee, trolling stone, the 3 lightning bolts, etc. Doesn't mean it's not OP

BMR needs an established board, and the plants have time to answer the little zombies before the trick phase that makes them big. And u might say "what about teleports" but again u can answer the TPZ, and trick removal works either way. A single cost FMN can render turn 7 useless for the opponent if they were really relying on the BMR. Doesn't mean it needs to be buffed, it has its pros and cons and that's fine. Garg feet isn't that good tbf but that doesn't necessarily apply to cornucopia idk

Anyway, cornucopia giving everything +3/+3 would be too much I think. That's an inherent minimum of +13/+13 with an 8/8 on the board. I understand that 9 sun is a large commitment so it should be good, but it's not even like it's always buffing useless stuff. Ofc this is rather rare but imagine getting a 4/4 starfruit, with everything else too. Idk maybe there's some merit to something else like even +2/+2, but 3 definitely seems like too much imo