I've had a lot of STX and STX-RC parts throughout the years and I never thought there was something wrong with them. Shifters, derailleurs, brakes, levers, hubs, cranks, pretty much everything!
I always liked it better than LX because of its underdog status and, with few exceptions, I always regarded LX as a "soft" group that lacked a certain something. No wonder it pretty much died off...
These being said, I will always respect STX & STX-RC (and the following Deore group, as a matter of fact) for its no-bullshit and down to earth approach. I wish more bikes came equipped with them.
The only true weakpoint of the group were the brakes with the fiddly canti-style adjustment.
Rear derailleur - no true difference to LX, just the color (please prove me otherwise!)
Front derailleur - I really don't have any clue about them, but what big difference can be there, really?
Shifters - LX were a bit fancier what the finishing touches are concerned, but shifting was always good with STX and STX-RC
Brake levers - almost identical to M600 LX and red DX
Brakes - like I said before plus the M600 brake started all the rip-offs and I am glad it did because today it means "brakepower to the people!"
Crankset - LX was better here and I guess better materials were used; 5-arm cranks are from a softer alloy for STX and STX-RC; 4-arm era cranks were better in STX-RC's case, but LX was close to XT in look and functionality; weight couldn't be far off
Hubs - same stuff different colors, if you ask me; no "suspension" option, but who the hell needed that and STX even had the Silent Clutch option
Combo-levers also looked very similar except that STX and STX-RC units were limited to 7 speed and when STX-RC got 8 it was only for a short period. I just cleaned a set of 1994 STX's and put them on a bike - still working fine!
That's my take.
Mx