Last Sunday, a friend and I were playing a game of Alpha Strike, and in my formations, I'd fielded a Phoenix Hawk LAM. The LAM has some definite pluses:
1. Transformation is instantaneous at the beginning of movement
2. Uses WIGE movement in Airmech mode (including the +1 To Hit modifier for being airborne on top of a +4 or +5 TMM)
3. Suffers damage and criticals as a BattleMech rather than as a fighter
4. Does not, apparently, suffer the "used Jumping movement" to-hit penalty for the LAM's attacks in AirMech Mode.
In short, by how we were interpreting the rules, the LAM could go almost anywhere it wanted and pretty much backstab any unit on the map with relative impunity.
Neither me nor my friend could find any drawbacks to balance out those pluses, other than WIGE terrain restrictions. I'm just wondering if there are any drawbacks to Airmech mode that we're missing that would induce a player to want to go into BattleMech mode.