I had an '81 for about two years - GREAT CAR! The only "show-stopper" problems I had:
1 - The Bosch fuel injection system ('81 was first year for it) was temperamental. Driving down the road, the engine would stop and start, stop and start because the FI wasn't pumping enough fuel.
2 - The clutch would freeze up in the winter.
Other things to watch out for:
3 - Fiat didn't know what rust prevention meant. If you can find one that's still in one piece, I'd suggest having it rust-proofed and undercoated.
4 - The electrical system is not the greatest.
That X1/9 is the reason I have a Fiero today. The Fiero gave me the personality of the X1/9 and added reliability and easy to find/inexpensive parts. But you're right - the X1/9 will run circles around the 2.5