I believe the "problems" people have had with WRX trannys have been folks that significantly increased their HP. like 300+.

the stock tranny wasn't designed to handle that much torque. thus some people (with 400+hp) were going through trannys every other months. i recall one particular person went with a dog leg racing tranny which resolved his tranny failure issues.

I've never heard of a tranny failure with stock wrx or the 2.5RS