GM fiat is no different from any other tool. It's mis-use is the real problem here and the core of each gripe. I truly hate when a GM decides to change rules mid fight (haste) or table conventions are altered with no explanations (removing dead bodies upon their hitting the ground) or allowing 23 minutes to resolve a web spell's effect and then giving the next player a 6 second time limit on his turn. These are just a few of the examples of GM fiat abuse that I've been shafted with in a single game by a local 'GM'. This does not include numerous other examples that I ignored or that were inflicted upon other players (CR 3 magic trap with a 40 DC, disappearing magic items, one way anti magic shells at level 3, undetectable 'protection vs Good' zones [and 'yes', the player cast the spell.]. Needless to say, I no longer play with him.
A month ago, a fledgling GM had a case of the dreaded and party killing Die Roll Disease that had 6 level 4-5 characters on the ropes vs 4 War 2 orcs, an incidental encounter. The party scored 4 hits and the orcs made all saves for 2 rounds versus 7 hits and 5 confirmed crits, resulting in 3 party members going down. He called time out and called me for advice while the party thought about the next round. He went back and gave the inept Fighter a couple of pointers on how to control the terrain and trimmed a hit point or two off the orcs.
Both were blatant fiat abuses, no doubt, but I can't help but see a difference.