Reminder: Pantheon and Tryndamere are still in desperate need of a rework.
There isn't much to say here. Anyone who ever plays top lane ever knows exactly what will happen when facing these champions regardless of who you choose.
Pantheon: you will lose your lane. There is no stopping this, and he will have double your cs at the very least (even if he only has 40 at 10 minutes). If you somehow, miraculously don't lose lane, he basically autoloses the game for his team. So fun for everyone involved.
Tryndamere: he will only ever leave top to possibly get a rift herald. Even then, it's questionable. you will beat the shit out of him til level 6, and then you'll just farm until you group and he does not and pray your team can take turrets faster than he can. Bonus: your bot laners who never play top will see he's 0/2 and ignore he has 200 cs at 20 minutes. They'll try to stop him from taking your second turret and get dove and double killed.
Binary champions with no place in league of legends. Unfun to play as, against, with, etc. They need to be given fail conditions and loaded with strengths that can be played around.
And in case you're going to say, "oh no, you just got shit on by one 11!1!!," no, I actually just won a game against pantheon. I took ekko top into him and gave him 2 kills (when kha ganked him, not solo), but prevented him from dominating me. He had a 30 cs lead at 10 minutes, but because he was not super fed, he was basically useless against the rest of our team after lane. Felt shitty for both sides. The only reason he didn't take my tower was that my jungler WW ganked and held him off a few times.