Conjuration summons are genuinely fragile compared to actual followers, yeah. They have way less health and armor than Lydia, and the AI doesn't position them well - they just run straight at enemies. But it's not unviable, you're just using them wrong for tough fights.
The real trick is that they're better as damage dealers than tanks. Don't summon them into a dragon fight and expect them to survive - summon them when you're already engaging, or better yet, use them to pull aggro while you nuke from range with destruction spells or a bow. Frost atronachs are actually decent because they can slow enemies down. Dremora Lords hit hard but yeah, they die fast if you're not careful.
If you want them to last longer, grab the perks that boost their health - there's one in the Conjuration tree that augments your summoned creatures, makes a noticeable difference. Also make sure you're actually leveling Conjuration past Expert if you can; higher skill = stronger summons. And honestly, if you're on Adept, dual-summoning (summoning two at once with the right perk) trivializes most fights because enemies can't focus everything on one target.
That said, if you want actual tanky followers who survive AoE, you might just want to grab Lydia or recruit someone else instead. Conjuration is more about action economy - you're not replacing a tank, you're getting an extra body to deal with while you do the real damage.