Oak needs to go with all health upgrades if playing assassin build, plus upgrade pen as soon as possible its the only way to out-live spit and hacking of claws, plus shield = tough opponent. I have beaten oaks but the best can sometimes get the upper hand. I don't know the other builds.
Erebus can be harassed pretty easily IMO. So can sedna, but she shrugs it off and might pounce you, i recommend holding off 1v1 with her until a UB gets Foul Grasp for the stun and extra damage. Stunning the heal is hard, but I have done it, some of the time from luck but sometimes from recognizing a heal situation.
Reg can do mark of the betrayer and throw mines in the path which makes it a pain to want to go through with, but at best a reg is just gonna have UB wait until troops take mine damage and just spit anyway even with MotB.
QoT, watch out. UB is the last place you want to be, no joke. Shields reduce so much faster with spit, plus any reinforcement or a second DG and GG. Plus, spit regens fast and any good UB will be carrying mana + mana regen items specifically Heart of Life, so your limited damage is a bad combo.
Rook best way to avoid spit is towerfarm, but still not very effective for repelling spit. Melee damage yes, but I have killed rooks before with all spit who had ful towerfarm planted.
Against other UB, just mirror their actions if possible. If he spits, you spit. If he attacks, you attack. Trust me when I say that dying once to another UB is educational and beneficial to you, because then you will know if you can keep looking for him to kill or not. He will no doubt be determined for another shot if he loses, due to human ego. If you lose to him, now you know who to avoid.
And btw TB's, ring of fire does not prevent people from moving inside it, which is the way some of you play it as. Against rook maybe that strat works, but UB is fast and takes little damage to give you a huge amount of spit. Any experienced player will know if they can withstand the initial fire TB damage burst, or the effects of ice TB, and if they can survive it then a UB will be able to put a fresh batch of spit on ye and start hacking with some claws. Slowed movement speed is delicious at this point, plus no movement upgrade = Smote