Well, that's part of the problem with DotA (not that it's worse than any other MOBA in this regard): there's a lot of heroes with grossly inconsistent viability in pub vs pro, and I feel like things have been getting worse in that regard. Careful play can shut down Slark, but really a sentry ward only locks down an area. This is why dust is so often preferred for everyone else. A Bounty Hunter can realize there's a ward in an area and book it before he's completely iced (remember, so many invis abilities have a speed buff component), even more-so in a pub when not all 5 heroes have stuns. Again, I agree Slark is terrible because his end game carry viability is pretty much non-existent, but the rate at which there is an early game runaway Slark effect in pubs by players of average competence is ridiculous because he's so easy to play. Phantom Lancer is another one of those heroes that has always annoyed me by how strong he is in pubs. I just think the heroes need to be designed a little more mindfully so we don't have all these OP pub, shitty pro heroes.
And yes, Shadow Demon is AMAZING. With amp damage you can lane with almost anyone and get kills.
The game is design with the mindset of "if you don't want this hero to get out of control, counter-pick him."
I play -AR. My last game we randomed 5 hard carries. 4 of us repicked and got no improvement. Our final lineup was Void, Sniper, AM, Pudge, and Riki. We lost to a PL. That is a risk you take playing AR. It's a risk you take playing in 95% of pubs.
I really don't get your complaint. Slark actually isn't an easy hero to play well. He's laughably bad before level 6, and most pubs miss his leap. Yes, he gets a free Heart when he is out of sight, but that doesn't make him OP.