Against Ultra Beasts, beast balls are 5 times as effective as ultra balls, so they're definitely the second best Poke ball for catching Ultra Beasts (second to master balls, of course).
YES. I mean, they are given to you for free, so you have that.
It's near impossible to catch an Ultra Beast without an Ultra Ball (And Vice Versa)
But if you want to be an absolute showoff, you can catch them in premier balls. (It IS possible, just a VERY VERY Low catch rate)