I can't agree with the opinion that Felon is too strong.
Instead, I believe that Felon is in exactly the right spot.
The Shield Factory without Felon/a weakened version would simply be bad. As already mentioned by some, Felon carries shield balls . And shield balls are simply the reason why you play this factory. Nothing else, neither Racketeer, nor Roaches or Aspis are good enough to justify building this factory on their own. Basically nerfing Felon just burries Shield-Fac.
Of course shield balls are strong. But they often only really become so when you have already invested a lot of metal. I think that's fair.
In addition, most factories have relatively good/easy counters.
Without going into counters that are not quite so simple:
Bot factory: Slings
Shield factory: Racketeers
Rovers: Wolverines & Impalers
Amph: Bulkhead/Buoy
Heavy tanks: Tremor
Hovers: Penetrators
Gs: Nimbus
Air: Likho
Of course, some of them only work until the sb has reached a certain size, but this is also the case after thousands of metals have been invested.