Tychus is literally designed to ignore the problems of focusing a high-health target. His entire gimmick is “practically ignores armor and numeric health”. He essentially has an automatic 20% damage bonus vs. Cho’gall thanks to the %HP damage ignoring armor.
The big key NotParadox brought up was that tanks aren’t that much harder for teams to burst away, virtually no character can solo burst a tank to death, and Sylvanas is no exception.
Because you’re otherwise unlikely to actually kill anyone off the burst alone, and landing the burst on the backline is rendered quite difficult with Sylvanas’s limited range on triggering AoE functionality. She has to take rather large risks to use this burst, especially outside 1v1 or hitting a tank with ti, ones that are best resolved by not taking the self-sustain talent at 16 and instead grabbing a replacement escape ability.
The particular problem being whined about isn’t actually much of a problem, competitively. Casually? There’s far more egregious things than Sylvanas around, in terms of reliable burst damage. Practically the entire build has to be dedicated to this burst, while many others can get away with just a couple key talents to improve their damage output on much more reliable things.