Thought I'd chime in here. I'm a ok to good UB player. I've played all the demigods though hard and won if that matters. My ranking online isn't too shabby.
The reason people get pissed off with UB beast is they don't equip properly to counter him. A good UB player will completely tear apart any noob player. But thats true for any Demigod pretty much. As stated above UB is one of the easier gods to learn and maybe thats why we are seeing all the moaning.
Ok so how do you counter UB? First realize that a spit+ooze combo is a one trick pony. A good UB player usually picks ooze OR Spit. Picking both gimps you. UB will either go health or damage... health being the most popular. Here are some strategies based on various DG's which do frustrate the hell out of me as an UB.
1) You see UB, pick a speed build. Speed is your friend. UB has no range attacks (outside of spit.. and spint isn't that far). Just stay the hell away from him early game, decide when to engage mid and late game.
2) Reg: You have the range advantage. Keep your speed up and you can't be killed, all the time doing damage to UB. Stay away from UB beginning and mid game till you have upped auto attack and gone glass cannon with +crit items. A lvl 15+ Reg with crit miltplying items tears UB a new one.
3) Rook. A Garden tower rook wins against UB. Every time. I hate rooks when I play UB. If the rook is smart and goes Tower build, I can't beat him. Again, this is all solo... this is a team game. Any 2 DG's can easily overwealm a equally leveled solo DG every time if played correctly.
4) Oak. Oak is squishy beginning to mid. Late game oak is unstopable. See the pattern here?
5) Sedna. A heal sedna can't kill a Ub, but UB can't kill Sedna either. Hold till a partner comes. Pull the UB back into a tower if you can (and hes that stupid) If sedna has minnions the UB will most likely go after the priests first then the others. Keep mana to summon new priests and heal yourself. Its a complete standoff.
6) Erebus: go + mana items, go mist build. You'll force UB to move away. You might not kill him, but he won't kill you.
When you play against UB your TEAM's job is to stop UB from leveling. Be fast, be smart, don't die. UB is weak against towers and ranged attacks. Don't go toe to toe with him. A good UB won't engage in range of enemy towers. If he does, hes usually in trouble, or hes confident hes got enough health to outlast the damage. If hes confident, pull him into the tower, have a waiting partner pop out of the fog and slaughter him.
In summary UB is a very strong 1vs1 player but he can be countered 1vs1. In a 2vs2 game UB is the weak part of the chain since he doesnt lend alot to a team. The most devistating 2 player combos don't include UB.
Oh and to adress the OP,late game is all about items. TB, Oak, Reg for instance will toast UB at lvl 20. Go heavy burst damage and UB is in trouble late game. UB's ability damage is all done over time. Its easy to beat him wehn you realize all you have to do is engage from afar (or near if shielded with oak) and pile on the burst damage. UB is screwed when hit with burst damage at high level.