Playing on T2 with no particular rush, doing some side quests, dungeons, etc, and trying to achieve +2 skills from paragon from most areas I hit Lilith at about 50. I wouldn’t call it super safe, just not in a big hurry.
So I spent a couple of hours in town to upgrade and tweak my gear at my “big 40”, and I was strong. Or so I thought before facing level’ed up mobs 
felt like a breakpoint. Would make sense to give them extra buffs at those levels, because you got healing potions improvements for instance, so maybe blizz wants to over(compensate) a bit? There have to be spikes in mobs strength as it is in your, because game feels very balanced overall, you really don’t feel that it’s a low level zone if you come back to some level 1 side quest. BTW fought the rogue in the den at 40, and it was soo much harder than doing that quest “on curve” level 15ish. TLDR: might be best to fight bosses at x9 instead of x0 levels. Also at the points when your upgraded gear spikes you a bit. Sometimes you want to wait because you get access to important talent point, or again important gear, so its complicated so far and I am loving it
If they made that ability a one shot instead of the floor falling, you still would have lost your character.
You get a good warning about the floor being bad.
Don’t stand in bad stuff 101.