When it comes to scaling, I keep thinking about 3 games: one did scaling, and it created a bug that was really annoying. The other one did scaling, but not in a very noticable way. The last game does scaling in what I think is the best way.
When I played the Witcher, I had scaling on. That kept the challenge up, for sure, but at some point I ran into a quest where I had to survive an attack from rats in the sewers. That quest literally wasn't doable because the rats were the same level as me rather than the level 1 they were supposed to be. I'm not sure if that counts as bad scaling, but as soon as it breaks the game, I prefer not to scale. I also agree with OP, what's hte point of achievement if it doesn't get easier to beat enemies?
The second game I think about when scaling is TES5: Skyrim over Oblivion. I've modded that game till it broke, but I'm pretty sure it's vanilla that when you grow stronger, more of the draugr would turn into restless draugr, and then up a notch more. I'm not too sure about this though, it's also possible the strenght of the draugr was just based on dungeon level.
I do find Skyrim a bit confusing, because it's technically open world, and you can go anywhere from very early on, but there certainly are areas where the enemy outscale you and you have to increase your level in order to fight them (especially giants come to mind here).
The last game I think about when scaling, and I think that this is a very interesting one, is Shadow of Mordor and Shadow of War. The enemies in the games itself don't scale, the very first orc you fight is just as strong as the last one, but captains keep getting stronger during the game, based on your actions, and your deaths.
Personally I found Shadow of Mordor alot of fun because all the skills you get, which makes fightinng later on more of a show rather than a challenge, but the fact that you missed a challenge (other than the occasional strong Captain) could make the game boring.
Out of those 3 games, I enjoy the scaling in Shadow of Mordor and War the most, mostly because you do get an achievement by getting stronger and killing those low ranking orcs easier and easier. It also has the most immersive way of scaling "bosses". Each time a captain kills another captain, or kills you, they gain strenght, therefore scaling.