Fisher has I think six winning seasons in two decades (off the top of my head, without referencing).
When I looked 50 deep among active and former HCs with the most games coached (his body of work and resume has nothing if not longevity), he was something like in the bottom two regarding career winning percentage. Not good. He has inherited bad teams twice and the Titans went through a salary cap purge (though he may have contributed to some missteps that brought it about in the first place, sometimes hard to disentangle what was the doing of Fisher, former GM Floyd Reese and even hands on owner Bud Adams in HOU/TEN - pretty sure he is calling the shots in STL), but even after being established and installing his system for multiple years, he has proven capable of little more than consistent mediocrity. If you want to take a team from 1-15 to 7-9, he's your guy. From 7-9 to 11-5, not so much.
They have been decimated by injury, but so have a lot of teams. I still expect him to survive this season, but in the past month, I'm concerned the team has quit on him. If true, obviously that would be a terrible sign. Other than McNair, he generally hasn't done well with QBs, but maybe that is one reason GB, PHI and NE were consistently competitive (Belichick and Reid also with the most career games among active coaches?). GBs Ron Wolf had a philosophy of drafting QBs nearly every years, and he was able to flip QBs like Mark Brunell and Aaron Brooks for draft picks. He ACQUIRED Favre via trade, than drafted one of the best QBs of his generation in Rodgers when 20+ other teams passed, despite having Favre and needing to park him in the garage for three years (for all they knew, it could have been longer). They value the position, and so will likely have historically great starting QBs Favre/Rodgers more or less continuously for a third of a century plus. NE and PHI flipped QBs for second rounders in multiple instances between them (NE got Brady in the sixth, they seemed OK with former #1 overall Bledsoe).
In the past 2-3 years, Fisher needed to start a proven non-starting caliber QB for a significant part of the season in Kellen Clemens. Backups thrust into starting roles in that span such as Hill, Davis and Keenum have one thing in common. They are UFAs. I get that he prioritizes the run, but if he had at least a marginal or competent QB, it would make the job of the offense in general and Gurley specifically a lot easier. He has also, as noted above, seriously bungled the OL in his STL tenure. Injuries happen at QB (especially if you have a lousy pass blocking OL, they are almost inextricably intertwined), and his contingency plan Bs in STL at the most all important position in the game have been impossible to characterize even by the most supportive observers and sympathetic critics, without sugar coating it, as anything better than heinous.
The current top five in Rams salary cap: Quinn, Long, Cook, Saffold and Kendricks, other than Quinn, have had very little impact, so imo that has been grossly mismanaged. Fisher has contributed to his woes, it isn't all about injuries.