You're right. This is just all hypothesis. But in conjunction with a hypothesis comes research and from my research plants are nearly unbeatable across the board right now. If, with my experience, research, and unlocked cards, I am only winning 1 out of 10 games as zombies then I have no choice but to see plants as overpowered. There are definitely counters to every plant card, strategy, and mechanics but on the zombie side those counters are overpriced, or so specific that you open yourself up to even common card strategies.
IN MY OPINIOION, from playing both plants and zombies, the people that are pushing back on the idea of plants being overpowered fall into the following camps:
- pure plant focused players
- new players who have limited cards unlocked
- or the infamous internet troll (probably a combo of the above)