I've been driving my Fiero with the L67 Isuzu 5 for almost 2 years now without any issue. I drive fairly conservatively, meaning I don't launch from a dead stop that often, and it has held up to everyday driving quite well. the only problem I've noticed is that first gear is a little short, but other than that if you drive moderately you will enjoy it so much more than the 2.5l boat anchor.
I briefly owned and drove a 3800 SC series II Fiero with an Isuzu five speed transmission. I did not beat on it and did make one trip from Chicago to Atlanta successfully (after the master/slave cylinder failures on the initial try). I did not drive it very much and only purchased it for the engine which I planned to swap into an 88 Mera. However, plans changed, I went with a 383 SBC in the Mera, and sold the car to another PFF member so that he could do a swap. He hit some financial problems and also sold the car so I do not have any long term reliability data for the vehicle.
I ran an isuzu on a SBC that dyno's 282 whp and 317 wtq. I didn't do many burn outs, but did lots of WOT runs with in 2nd, 3rd and 4th gears. It never gave me any issues, but I also swapped to a Getrag about 2 years later.