Problem with tank Karthus is your q and r suffer a lot from it. And your passive becomes pretty much straight useless.
And you're only kind of a mild disrupter with your wall. Yeah, sure, you'll survive a long time, and have mana to sit there with your e on in the middle of a fight. But you can largely be circled around and ignored (by a smart team) cause his ms is bad, and no dashes or speed boosts of any kind.
The q is the bread and butter of it.
If you're good with q's, you can stay REAL safe (unless they have a Zed or something along those lines), so the fact that you are glass cannon means very little, unless you make a mistake and overextend. Plus in late game you can pretty much just yolo baron pit and die and still win a game off it, so he maintains a huge threat all game.
Your E passive returns mana for cs'ing. So if you're decent at last hitting, and at least get a
or
early, you shouldn't have much mana problems, unless you just continually forget to toggle e off.
That all said, I think Karthus is in a pretty bad spot in the current meta, along with the last 15 or so meta's.
He's not bad, but there's almost always better choices for anything you want to do.
all kinda just do different things his 'job' can do, but they fit better in almost all team comps, where he's kinda an outsider/"niche" for...something that doesn't really exist anymore.
He's a dps team killer that can't stick to targets, and is pretty bad at contesting objectives.
Even the super late game baron thing, it isn't even that he'll be all that likely to contest the baron, he'll either ace the enemy, or not. While acing them super late game will win the game, games usually don't get to that point, and a smart enemy will just end the game some other way than try for baron.
And objectives have been WAYYY more important than open dps fights for a long time.
But if the enemy looks like they're going for a hardcore teamfight comp, fuck it, play Karthus and wreck them.
But still gonna vote glass cannon being overall more useful than tank.
Unless your team just really needs a tank...in which case...why pick Karthus to begin with?