You've only really got two that aren't 100% uptime and those aren't big enough magnitude to cause you to wipe. Just don't press the blue buttons and you're good. Pressing everything else off cooldown is perfectly fine. Yellow Captain has a whole lot of filler. Granted if you do go into that run please tell them that "Yellow Captain is a high skill ceiling specialisation". I think they could do with a laugh.
It's already been pointed out they're fine in Stairs. They're technically not going to struggle that much in Remmorchant, they can execute the tanking part reasonably well but people have this weird expectation that your tank should be able to halve the damage everyone receives during things like blood surge. Not that anyone is gonna try that out since Captain makes things so much easier and nobody really wants to put more effort into doing a run than they have to. Same reason why you won't really see anyone other than Captain boss tanking AD, it's not that they definitely can't...it's just people don't want to take anything less than the broken OP spec. Heh, block people from speccing yellow Captain and the progression race for T5 might even last longer than a day.
Technically a yellow bear would also work for external defensives, yellow sacrifice being fairly powerful and all. But you know, heaven forbid healers use cooldowns on tanks to help them survive. We are totally better off just having tanks be entirely self sufficient and ensuring healers having nothing outside of rotational heals to bail tanks out in tricky situations. That scenario feels great for all involved!
Yeah, I know that Blue Captain has the highest sustained AoE hps of any spec. I mentioned that earlier:
Have another post somewhere mentioning that their AoE healing does need some adjustments but you do get overly upset at the mention of the word "nerf" so I thought it would detract from the point at the time to go into detail about all the changes blue Captain probably requires. My bad I guess, although said changes likely mentioned removing revealing mark so you probably would've begun to froth at the mouth.
You know, I can't help but notice you didn't explain what these miracle buffs for all the other tanks that make them competitive with Captains would look like. Makes me wonder if you thought about what would be required to level the field...and realised that such buffs sounded utterly ridiculous.