So I don't think jack drops are too op. 1 Jack costs 600 Metal + Charon (100) that's 700 Metal.
I think it is simply the fact that certain units are better / less effective against certain units.
I would say almost every factory has a decent counter against jack drops.
Since you have already made a
Post about Jack Drops to op, I have given you examples of how you could react according to each factory, note Jacks have low range and can jump:

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= Swimming Jacks lulz

= no "early game"