Majority of any set is not going to see play. It was true for gvg too, yeah there were like 30 or so strong cards and the rest were just awful or gimmick. Awful cards are necessary any ways, they design a set for arena or new players too.
This has been true so far for both TGT and GvG. I'm not expecting the new set to suddenly consist of 50 percent competitive cards.
Now for TGT the percentage was really low and those cards that saw play mostly weren't too strong but good and balanced I would say except the ones you know.
Average quality of legendary cards in TGT is way better than GvG imo. GvG cards are mostly gimmick fun cards and then there were Boom, Mal'Ganis and Vol'jin. Maybe Toshley too, the rest were gimmicks or just awful. TGT legendary cards are generally playable, maybe slow but they have reasonable effects. Cards like Rhonin or Mist Caller or Eydis twins. The only bad ones are Wilfred, Skeleton Knight and Bolf but even those aren't as useless as Hemet or Flame Leviathan... There were no Dr. Booms (good thing) but on average they are pretty good. They could see play in standard.
I actually hope they keep the power level of card on a similar level to LoE overall. There are strong cards but you have to build your deck around them. They can make exciting cards that aren't just crazy broken.