Mech vs. Team-ups
This has been brought up before but I was working on my Imp in Weirding Woods and it brought it to sharp attention. While the mech isn't actually bad, it pales when compared to the Oak and Station:
-It doesn't heal like players do automatically, but the Oak and Station do. The Oak makes sense in a way as it's a tree but the Station like the mech, is a machine. When you add to this the fact that the Oak can have acorns ride on top and both heal and shoot, and the station can have cadets add shields and attacks, plus the extra health, the mech doesn't fare well.
-The mech has a timer but the Oak and Station do not. In fact jumping in and out of the Oak and Station is a common tactic in both PvE and PvP. Jump out, go heal or get healed, then form back into a full strength team form and return to battle.
The team up idea is really fun and Space Cadet may be my favorite character in the PvZ shooters to date, but currently the team ups are just too good compared to the mech. It can tie up to 4 people in one target, and they're all big targets, but the advantages of the new team up gestalts makes them better than mechs. And as posted elsewhere, especially for Oaks, 4 Acorns could just make 4 Oaks that all operate independently, naturally heal over time, and can jump out and evade to heal to come right back when threatened with no penalty.