I played a fort build straight through to glowing cops.
On default difficulty, it's AMAZING....aggro a whole room, move to a choke point and start normal swinging....your fists hit so many zombies, and with so many proc abilities it's like a slapstick comedy skit watching the zombies fall all over themselves. The crowd control of the knuckles is amazing (why are you at gamestage 130 and your main melee style is only 4/5?), and when you need to dart in and kill a spitting cop, you have reliable enough dismemberment to sprint in, normal attack the head several times, and sprint out to another bottleneck.
Natural healing, plus bandages, plus fortifying grip, combined with the damage reduction perk and steel armor, and the crowd control of the knuckles, and actually standing in a marginally intelligent spot instead of surrounded is more than sufficient protection - and 5/5 fists, using NORMAL attack (power attack is for solo fights and early levels) rapid firing 6 different procs Brawling 2, brawling 3, brawling 4, brawling 5, weighted head, burning shaft) on several zombies at once with every single super fast swing can handle most crowds easily....unless you decide to fight in a wide open field.
I felt like an unkillable tank...which seems exactly the point of the fortitude build.
Now, if you read this and say "of course you felt invincible, it was on default difficulty!" - well, that's the point. The game is balanced for default difficulty. You start changing the parameters of the game, and of COURSE some items will outperform others. Set player block damage to 300%, loot amount to 300%...and I'll bet those ammo-chewing M60s look even MORE attractive. Set difficulty to minimum and zombies to jog at most with loot % 25% and the hunting rifle really shines!
it's not viable for anything.
I can attest the fortitude build is PERFECTLY viable....on default settings it accomplishes EXACTLY what it implies that it will - you are a walking tank, and you outlast your foes, and all those stagger/knockdown/ragdoll/beheadings you were promised absolutely happen. If you want to play the game on the wildly different settings allowed in vanilla (or modded outside of vanilla), then maybe don't expect everything that works on default to work exactly as well. If you are already playing on default, then try 1) actually maxing the skill, and 2, fighting in a setting and manner that utilizes the particular strengths of the melee style you are currently favoring.