Tomb of Levistus is terrible?

[Zanthy1]

I imagine because you lose a lot of turns if you die.

This for sure. I'm pretty sure the Tomb itself has 10 temp hp per warlock level, so thats 100 damage it can absorb at level 10. Sure at super high levels thats like one big hit, but using an action to absorb 100 damage is nothing to shake a stick at.