The current scaling is: (slightly) from level, stat gain, magic items. I am pretty much fine with this, although I think they should choose either stat gain or from level, instead of having both.
There is no such thing as bounded accuracy when you have +1 at level 3 and +9 at level 12. It's a huge difference which basically makes the level range of appropriate monsters too small for my taste, just like it's in 4e.
Let's take something with AC 18 at level 3, you need to roll a 17 (20% hit chance). At level 12 you hit it on a 9 (60% hit chance). As a DM I wouldn't use even one of the monsters at level 3 because just hitting 20% of the time is something that just gets frustrating as a player since you won't be able to do anything to the monster 80% of the time.
Instead I think adding to the hp - and damage as characters level and monsters get more dangerous works pretty well. A could then use maybe one of the monster at level 3, five at level 7 and eight of them at level 12 to challenge the party.
It also makes it so villages and towns without lots of high level adventurers can protect themselves (at great cost of lives) since even bands of ogres, giants and such can be hit and taken down if there are enough level 0 hunters with bows. In 3e level 0 hunters would stand a chance against any giant, since they wouldn't be able to hit it other than on a natural 20 and because they did very low damage.
tl;dr
In short, I think having nearly no scaling of attack bonuses, and relatively low scaling of hp, more in line with what you had in AD&D, will result in an environment that makes a bit more sense than the current 3e and 4e worlds where commoners just have no chance, no matter the numbers against a level 10+ party.