I haven't run enough DF games to have a strong sense of this. We also tend to customize the challenge to whatever the players want. My first DF campaign last year didn't have any tanky types, so the cleric was often at the front lines along with the swashbuckler.
I'm currently playing a cleric (
Zafir Abrashi) who is doing ok in combat. He'll be better once I get some fancier armor. I did just get Heroic Grace which should boost his flail skill to a level that permits better targeting of hit locations or deceptive attacks. I was torn between that and Combat Reflexes but decided to focus on offense since he can always boost defenses with his Armor and Shield spells.
In principle, I don't have a problem with Holier Warriors (and Thiefbucklers). One of my players wasn't happy with his pure thief. I wasn't sure if this was because he was legitimately underpowered or if the player just wasn't into the archetype (it was a pregen he picked up when he joined midway through).