From eyeballing it, and DMing a Pit Fighter, I'd say they probably are great at low levels and then lose out by the time mid-levels roll around.
Pit Fighters are great at standing back up from hitting zero hit points, but a Fighter will ultimately get hit a lot less and -- more importantly -- will kill the monsters a lot faster.
Yeah, testing against a Chimera at level 10 it doesn’t look great.
Pit Fighter with 18STR, 14CON, DEX 10. Leather armor, shield, bastard sword 1h.
2 talents to increase strength, 2 to +1 melee damage, 1x ignore damage from one attack
On the damage side it is d8 + 2 so 4.5 + 2 = 6.5 hitting at + 4 so hitting Chimera AC 16 at 12+ ie 45% so effective 2.925 damage per round.
Incoming damage is 36 rend and 14 fire hitting AC 13 with +7 on 6+ ie 75% and making DEX15 with +0 30% for 27 rend and 9.8 fire for 36.8 per round.
Starting with 6.5 hp and gaining 4.5 nine times leads to 47 hp. This means the Pit Fighter goes down in 1.28 rounds.
Flourish applied once adds 3.5 hp takes that to 1.37 rounds. Immunity to attack negates one rend for additional 9hp taking us to 1.62 rounds. Succeed on DC18 con check on 16+ 36% of time with advantage which we will count as 0.36 of one 9 point rend for additional 3.24 for effective 62.74 hp and going down in 1.7 rounds.
I could apply another partial flourish or factor in the 18+ on revive but I’ll stop here. Doesn’t look great with issue being low AC. Ironically this means a Pit Fighter should go +1 to melee attack over STR/CON and focus on Finesse weapons and favor DEX.