Thunderbringer is a hero that has remained largely unchanged since his introduction as one of the first heroes in Dota. His skillset has remain basically the same throughout metagame changes, introduction of new items etc.
I must say this hero is currently incredibly underpowered and quite possibly the worst hero in the game next to shadowblade. This hero has become the definition of obsolete and needs a major change. This hero has so many deficiencies i dont even know where to begin.
First off, he is an int hero that fills more of a semi-carry role. He can't support (no disables) and needs levels and farm. With this he preferably needs a solo lane like mid. The problem with this is that this guy is made of paper. Hes probably the squishiest hero in the game. He will be the first target to die in any teamfight and has no way to stop it.
He has no way of escaping ganks b/c he is the only INT hero in the game without any kind of slow or disable. The only others being Defiler, SR, and vindi. Of those 2 have silence and SR has a heal. Defiler and SR also can focus on building tank, while vindi does not need levels or farm to support.
Mana battery + vestments pretty much counters TB's earlygame, and these items are cheap and stable pickups on almost every character in the game. TB is far from a mid stomp int hero (tort, poly) and is food for popular mids like tremble, nomad, Fayde. What makes this worse is that hes a high risk hero, if he falls behind earlygame with 1-2 deaths he quickly becomes a liability as he does not have good mid-late game farming ability. Compare this to other int carries (SR, Defiler, Puppet, tort, DR).
While on paper his 2 nukes look good, his burst is very overrated. This is simply b/c he has no way to keep on target. If you run up on someone you can get 1 chain off and 1 blast off and they will simply run from you. Compare this to other burst heros like DW, Fayde, Pebbs whole all have CC and can solo burst down a hero.
What makes matters worse is TBs poor attack range, average MS, and poor starting damage. At this point 350 range is almost a gimp and worst than melee if your not an auto attack carry. At least mq who is equally squishly early and has same Attkrange has 320 ms so she can kite her opponents.
To sum it up, TB is item dependent yet not a good farmer. He does not carry or farm as well as other int carries, and cannot carry as hard either. He has the worst survivability in the game and ironically is gimped the most out of any hero once he falls behind. Compare to a hero like midas, who has equally strong nukes, flash farm ability, an escape mechanism, CC, push ability, as well as survivability due to being str, I cannot fathom a reason to ever pick TB.