Just like how you finally saw Slark was not OP, maybe even without a nerf to Lina in 2 months you'll see that she's not OP either.
Nobody is saying she isn't strong. 950 pure damage ult with Aghs is undeniably strong. The fact that she can be played as a core or a support makes her useful in Captain's mode as a fakeout hero too. Is she OP though? 47.19% win rate suggests not really. Even at TI5 it felt like she never ran away with the game by herself unlike, say, Storm.
Lets be honest, she's totally going to get some kind of nerf. I think that's pretty undeniable.
My main issue with her is just she is so ridiculously frustrating to lane against. Unless you have Techies (just his attack range, not his damage, lol) or Sniper's (with Take Aim) range, good luck. Unless you counterpicked with Leshrac or TA, it seems like a lost lane.
Storm in particular is an example of a matchup I don't really see a way to beat her with, but this can apply to plenty of other mid heroes too. Her attack range is one thing, but then Dragon Slave's range is so ridiculous, and it's super wide too. I can try to anticipate and dodge it, and then of course she throws you for a loop and casts it in a different direction, despite the fact that she was right clicking down two creeps to obvoiusly try to get the double last hit out of the Dragon Slave.
The main issue I have with it is that pretty much no hero can take creep aggro to control the lane from her at all. She can sit there and do it from high ground and nuke you if you even try to approach in range to take creep aggro.
Puck - Shorter attack range, can dodge stun with Phase Shift, but Dragon Slave animation is too quick and you can't really anticipate it easily.
QoP - Probably worse off than Storm against her. Good luck getting close enough to Dagger without losing at least half of your HP, if not more.
DP - Not great right now in general, but she basically wishes she was Lina as far as laning goes. 600 vs 675 attack range. Crypt Swarm is pretty much inferior to Dragon Slave in almost every way. Presents a lot of the same problems as it is with other heroes.
Kunkka - Melee, Tidebringer might help by pushing creep wave into tower faster to get control back, but still you will have a difficult time taking last hits without Tidebringer.
Razor - Static Link range is shorter than her attack range, plus she has plenty of ground to run back to break it, and you will be forced back.
Viper - This one might work, would have to give it a try.
Skywrath - Can Bolt her from a good distance, also plausible, but you would have to Bolt for CS more often than you would like.
SF - Stands a chance, still in a tough spot though with how frail he is. If you can land good razes for double CS and hit her you can win it. If her positioning is on point though, good luck.
She beats almost every single one of those mids, and those were some I just though of off the top of my head, I know a lot of them are not the hot mids these days.