So his PC sounds like a disaster - AT/WM is a remarkably bad combination that is likely to put him on the frontline, with a probably-bad AC and apparently intentionally terrible HP.
Actually, he is rarely on the front line (shortbow sniper SA type), AC 19 (
mage armor, DEX, ROP +1--so not great but not bad at all), and average HP with CON 14. But he can front line, but moves in to strike and then back out. He rarely goes "toe-to-toe."
Anyway, the combination has worked out very well but it is just an 8/2 split, so heavily favors the rogue.
It is more the Rune Knight was an insane tank. HAM, CON 20, tough feat, Hill Giant rune (boy is THAT broken!), etc.
Do you roll for HP at your table? Because the fact that he had "about half" the HP an RK had is remarkable if it wasn't unlucky with rolls (or the RK was incredibly lucky). With no rolling if he was say, L10 and a 5/5 split, you'd expect him to have 45 HP with 10 CON, and an RK would expect to have normal Fighter HP - i.e. 60. That's only 33% more. For it to be 100% more, the RK would need a +4 CON (or +3 if you're overstating slightly) bonus and for the AT/WM player to have insisted on having +0 CON despite making class selections to indicate he wanted to be on the frontline! (Or I guess -1 or worse, but surely he wasn't that suicidal?) Which is a spectacularly weird and anti-success choice from the AT/WM.
None one rolls, or at least not often.
RK (at 10th) has 134 hp. AT/WM (8/2) has 71 hp, so roughly half. RK has AC 20 IIRC
Odd too that everyone else was down when RKs have a major ability that can only be used to protect others, and are generally pretty good at doing so.
LOL I don't know what RK you've seen in play, but the RK in my game was not about protecting others. The only feature for protecting others IIRC is Runic Shield (?).
Honestly it sounds like you are just having an issue that can occur in literally any combat-oriented TTRPG, even OSR ones. One PC has a normally-designed, not even min-maxed character (the RK here), and one PC insists on making an incredibly fragile character who isn't a glass cannon, because he can't even do good damage, and doesn't provide good utility or CC either (which is most assuredly the case with an AT/WM), so is basically a largely defenceless guy on the front lines. Obviously he's going to get downed a lot! It's actually remarkable his PC isn't dead from three failed death saves! The other PCs must be working very hard to keep him alive, and/or you as a DM must be cutting him a serious break.
While such scenarios can happen, that is not the case here--in fact you're wrong with just about everything in this case.
But again, this can happen in any RPG.
No, in this case it is the RK who is OP. The class is broken, honestly. Nerfing it a bit brings it back on par with other subclasses and classes in general. The runes are ridiculous IMO, but I've gone over that in other threads...
At any rate, the other players are all
very happy that PC is out of the picture, and so am I. It was ridiculous the crap I had to throw at that PC to even threaten him at all. Granted, he was a maxed-out PC but the subclass was the greatest issue. It is the first subclass from Tasha's I allowed (and the last!) because the theme appealed to the player and I liked the
concept, just not its' execution.