They're not OP in my opinion
It's simply people not exercising basic tactic and common sense.
Like a hale who keeps getting backstabbed over and over because he doesn't watch his back, so he claims spies are OP. Yet watching your back and being mindful of spies renders them virtually useless.
Market gardener soldiers are super easy to deal with too:
- Don't run in straight or predictable lines
- Don't ignore them the whole round, especially on open market gardener friendly maps
- Just like you watch your back for spies, keep an eye out on the sky because you can see them coming from a mile away
- If they're still very problematic, pick off the team in enclosed areas, not out in the open, then go focus them down when there's less enemies distracting you
- When you're waiting around for something (I.E. power jump cool down) stay under objects that block falling soldiers
- Rage them when they land before they rocket jump away
And there's a lot more you can do
They're annoying and a little on the OP side on really big open maps with big health kits but not impossible to deal with