I think a large part of the reason Liverpool paid so much for Alisson is that he is good with his feet, he will enable to Liverpool to play it out from the back better than they could with Karius or Mignolet in goal. That's what really separates the very good keepers from the truly great ones.
I'd say the prem has a lot of very good goalkeepers (Schmeichel, Pickford, Butland, Pope, Heaton, etc), but to take the next step up it's not enough to be a good shot stopper and be commanding on crosses and set pieces, you need to be able to play out under pressure, which is something really only De Gea, Lloris, Ederson and now Alisson can do properly.