Tryndamere literally defies every single balancing guidelines Riot has. It is baffling how is he even allowed to exist in his current state without getitng Eve'd until his rework hits. Let's have a little rundown, shall we?
RNG reliant: check and double check. He lives and dies by RNG. His laning involves jumping on someone and praying to RNG gods. Oh, and his main basic spell CD is RNG reliant too. Do you win or lose lane against Trynd? You can never say. Only RNG will tell.
Feels punishing to play good against: Karma once had a passive that gave her AP based on missing %HP. It got removed because, a direct quote for Riot: "You felt punished for doing well - as she would get stronger the more damage you do to her." Tryndamere gets the same kind of ability - except he a.) has it amplified by RNG and b.) can stay extremely low without dying to (ab)use it the most.
Low counterplay: His W. With a simple click of a button Trynd instantly wins trades against any melee champions.
Hidden/underappreciated power: Again, his W. Removing 80 AD from every person on other team with no form of counterplay? That's 14400 (!) gold of stats down the drain. For comparison, Sona's aura giving a quarter of that was considered too much of hidden/underappreciated power to be allowed to exist.
False choices: Guess what? W again. Honestly, this spell is broken beyond belief. Not OP by definition, but broken as all hell. Fightning is not an option because of massive AD penalty, and running is not an option because of a huge, long duration, non-decaying slow. I believe I recall Xypherous or Meddler once saying that they'd never implement somethong like DotA's Bloodseeker ultimate because of false choices. Yet we have Tryndamere's W unchanged for years.
Low interaction: Oh hell yes. Trynd's playstyle is: just push lane, ignore everything. If you get ganked kill them if they are 1 or 2 and/or escape with your invulnerability. Rinse and repeat.