For me, it was the opposite with Alistair and Loghain.
I've never had any problems with Alistair as a tank, since getting him so early meant I was able to build him up the way that worked for me: I go strength to 42 for the heaviest plate and then concentrate mostly on dex for avoidance, with just enough cunning for tactics, and have him always running shield wall and threaten. I also set him to use taunt liberally. His personal damage output was not so hot, but he was a beast at soaking it up.
Loghain, on the other hand, comes pretty late, and so I didn't have much wiggle room to play with his build. (The playthrough where I spared him was early on, when I wasn't using many mods yet, and at the time I didn't know about the one that lets you respec your companions.) As a result, while he had no problems holding threat, he died at least once or twice on most fights even with Morrigan keeping an eye on him for heals.
I think the key thing I was missing with Loghain was dex. If I remember right, he had mostly strength and con by default, and I've found stacking con to be almost useless. Nowadays, I even stack Sten and Oghren with a good amount of dex, and I've found it ups their survivability quite a bit.
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I've never had any problems with Alistair as a tank, since getting him so early meant I was able to build him up the way that worked for me: I go strength to 42 for the heaviest plate and then concentrate mostly on dex for avoidance, with just enough cunning for tactics, and have him always running shield wall and threaten. I also set him to use taunt liberally. His personal damage output was not so hot, but he was a beast at soaking it up.
Loghain, on the other hand, comes pretty late, and so I didn't have much wiggle room to play with his build. (The playthrough where I spared him was early on, when I wasn't using many mods yet, and at the time I didn't know about the one that lets you respec your companions.) As a result, while he had no problems holding threat, he died at least once or twice on most fights even with Morrigan keeping an eye on him for heals.
I think the key thing I was missing with Loghain was dex. If I remember right, he had mostly strength and con by default, and I've found stacking con to be almost useless. Nowadays, I even stack Sten and Oghren with a good amount of dex, and I've found it ups their survivability quite a bit.