Which is why you don't go wandering alone, there's a Beast lurking outside. For most part of the games, if you're close enough to towers, you're relatively safe even is UB is faster than you.
And mostly every demigod has answers to UB harassing.
Oak has shield, and his passive skill helps recover from spit too.
Erebus.. well, really, Erebus can run away from anything. But mistform negates spit easily.
Queen's shield negates spit.
Rook may have some trouble with UB. But if UB misses with stun, it'll get Slammed, and Rook is tough in melee. Besides, Rook can have his own towers nearby.
Erebus has range and mines. Erebus can harass UB back, just staying near towers is even more important.
Sedna has passive and active heal, and is just so annoyingly durable she shouldn't have that much problem with Spit.
I imagine Torch bearer could also have trouble with UB. But TB has his own slow and stun, plus good ranged damage, so with harassing it should go pretty even at best.
UB is very offensive demigod, but doesn't really have any defensive skills. It's just common sense to stay close to towers. Melee classes are naturally at disadvantage versus ranged classes, they should also be more deadly once they get to melee range.
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Edit.. took so long to write this Luv3rboy already pretty much said it all, in a tighter package