Have a Regulus standing buy with Marks and Mines.

Because if there's an UB in the field, I as a Regulus am not running around alone...

This is a good example of you just plain outplaying the other player. Yea UB is great, but im sure a TB with the right gear who knows what hes doing can kill him too. With circle of fire down and a fire nova thats pretty high damage, then a 1350 fireball could be enough to finish it.

I have enough HP to survive the burst damage from a well made TB at level 20 by level 5 (as UB). Granted, he'd kill me with autoattacks, but I could freely sit in the circle of fire, take the fire nova, take the fireball, and still be alive. Plus most of those skills are easily interrupted (and besides, ice nova + shatter + ice rain + fireball does more damage than fire nova, fireball, and fire circle, especially if you autoattack once before the shatter wears off and even more especially if your opponent isn't stupid enough to sit in the circle of fire for 10 seconds).

Basically, your TB build isn't even a good one for killing other creatures (you get more effects + more damage from an ice build; pure fire is good for taking on generals and towers, and that's it), and even with a good one HP stacking makes UB tough to kill with it even early on.

Normally, as UB, I'm not afraid of any Assasin at all unless he's some levels above me or he has company. They can stack HP but so do I, and if I can spit and engage at say, at a middle flag on Cataract they have to run or die. Actually I never seem to have problems against Assassins. It's the Generals that are a pain. Well, good players playing them. Mid-late Oak is baaad news, Sedna, I can't kill her alone, neither QoT bramble shielding again and again while the other DG pounds me. Let's not talk about master escapist Erebus with the best spit counter. Often I just find myself locking the flag before I gtfo to an adjacent lane.

Spit damage is mitigated by HoT effects, but fireball is straight damage.

how is that even remotely true?

time is time and damage is damage.

consider, elapsed time of 10 seconds. you have a health regen of 40 hps.

Scenario A: hit by rank 4 Spit

you suffer 1500 damage from Spit and gain 400 health from regen. net health loss of -1100.

Scenario B: hit by rank 4 Fireball

you suffer 1050 damage from Fireball and gain 400 health from regen. net health loss of -650.

how did regen mitigate spit damage at all? it didn't. you took all of it. 

the only circumstance where Spit's damage over time nature allows your regen to lessen the severity of the effect is if your health was at the level where an equivalent direct damage spell would have instantly killed you. so if your health was at 900 in the above scenarion the fireball would just blast you entirely even though the net loss is less than 900. however, the spit is still causing a net loss greater than 900 so you'd be dead. you would have to have a health of 1100 to survive the spit in that scenario, which would also be enough health to survive the Fireball so whats the difference?

One quick thing to Thrandiul; UB is not the worst against towers. Erebus is (he has lower base armor IIRC, and since he doesn't have any moves to use against towers, he can't even spit like [most] UB builds can.)

Erebus can use minions to draw the tower's fire while he attacks it without taking damage. UB isn't bad against towers, but he's still the worst until at least mid-game imo. Until then he's too squishy to attack it straight up and the spit damage can be mitigated by the building health upgrade, which only costs 600 gold.

Ok, let me change that: Assassin erebus is the worst against towers (I never play him as a minion master.)

UB's DoT is directly countered by oak, QoT or vampy.  If you're oak, pop a point in shield and dispell it the second he casts it on you.  If you're QoT you probably have your damage shield on constantly so it won't touch you.  If you'r vampy, pop a point in mist and I'm 90% sure it dispells it as well.  There's 3 heros who can pretty much negate his DoT.  Sedna can heal herself and others just need to carry pots or TP scrolls.  UB seems to be a good counter to Torchbearer and Reg but again if you have an oak or QoT on your team a quick shield will cause the UB to lose most of his early/mid game effectiveness with spit.

I'm not really sure how I feel about spit damaging buildings but imo its still too early to start throwing around the imba card on spit just yet.

Which is why I love Ooze so much! You can't negate it! In Mist, Erebus can't do anything to you (just run around to cover his retreat), Shielded Oak can dispell Spit quickly but he doesn't do anything (extra) against Ooze (although it does get rid of the attack speed debuff), and bramble shield is still broken down by Ooze damage (though I think Spit still affects Bramble shield, it's just absorbed.)

Ooze has a counter. a much simpler one than Spit in fact. you just walk away. its basically a melee range AoE, any character with ranged attacks and decent run speed can kite an Ooze beast. 

now thats not a fatal flaw by any means, Ooze can still be very good even against ranged harassing types if you know how to play it right (you have to sneak up behind them). just saying, you can really easily negate Ooze just by getting out of the way. its much easier to prevent damage from Ooze than from Venom Spit. 

Without a doubt.

I do have to point out, though... that UB tends to be the fastest DG in a given game. Getting away from either ability can be difficult.

Who can walk away from an UB that has a high base speed and a 20% modifier to speed after one attack? Nobody!

20%? from what?

rank 3 Diseased Claws gives 10%. 

its perfectly reasonable to escape from a UB. Warpstone, Cloak of Night, Bat Swarm, Wand of Speed, or someone using their own slowing effects before running. its somewhat harder to escape Beast than other DG's but certainly not impossible. and if you're playing a ranged attacker you might just be kiting and harassing anyway and NEVER be in range, particularly if its a multi DG encounter.