Depends on your rims in my opinion.
I've got Crank Brothers Iodines and run regular non-UST non-tubeless ready Minions and High Rollers (and just gone to HRII)... not a problem at all. Never burped and fine getting on the rim (I use CO2 to seat them though).
A friend had some XC UST rims (might have been Mavic, not sure). Burped and blew off all the time. Same tyres.
Skinny UST rims and chunky tubeless-ready / none-tubeless tyres just don't go. UST tyres probably work, but they're heavy and expensive.
Sealant - absolutely necessary. OK, maybe you'd get away with it with UST but it would still be a help especially with small punctures. And no, you don't need to faff with the wheels for hours to cover with sealant. Soapy water around bead for initial seal and a few scoops of sealant in, spin once, job done. It remains sloshy unless exposed to air, and as you ride it will coat the tyre inside.
p.s. Maxxis stuff seems to be working out most reliable from what I've seen with people I know running tubeless. Also their non tubeless is pretty much known to be tubeless ready, and they are beginning to mark some as TR now anyway, more so with the bigger wheel sizes.