Oblivion combined both, and did it poorly, IMO. Morrowind actually, contrary to what many think, does indeed have level scaling via unlocks. Around Seyda Neen, you unlock a Nix Hound encounter near the tax collector's body upon reaching level 2. He is always there once you reach that level. Likewise, the higher your level gets, you begin running into more Blighted enemies even when not in the Red Mountain region. It is true you can run into tough enemies at low levels, but this is usually because they're entirely static and hand-placed (So it will never be anything but them, like how Skyrim does the same thing) or the lowest level enemies of a certain subgroup can still whoop your ass (Daedra).
Oblivion's level scaling, like I said above, combines enemy powerups and new enemy unlocks. The problem is, all enemies changed in power according to you, which meant that you could literally beat the game at level 1 with no resistance whatsoever, or struggle through at level 30. The only incentive to level was to see the new baddies, but the fact that the old ones become so ridiculously strong was a counter-incentive.