The-Magic-Sword
Small Ball Archmage
Well, without level to proficiency, you would have to increase the level gap between a "boss" and a party, much like you do in 5E.
A Vampire Count, for instance, is a pretty common choice for a "boss monster". If we look at select abilities, we have:
Armor Class 27
Melee Claw +18
Dominate DC 26
Without level to profiency, the monster is brought down to earth, as it were (it's level 9)
Armor Class 18
Melee Claw +9
Dominate DC 17
Does this mean it is now suitable for low-level heroes? Well, no, since it still got 110 hit points with fast healing 10 and resistance 10 (against weapon attacks from low-level heroes). This is still insurmountable.
Very very roughly I would say the variant doubles the level range. If an encounter previously was deemed of "severe" difficulty when it was three levels over the party level, I would now deem an encounter six levels over the party as severe.
So yes, the dynamic changes. Single powerful creatures lose some of their edge. On the other hand, large numbers of previously trivially easy foes gain some edge.
I really don't see any game-breaking issues here (and I certainly don't see any magical hidden properties of the game that simply doesn't work without level to proficiency), but again, I haven't tried the variant.
I do see adventure-path-breaking issues, however. It isn't as simple as "previously hard BBEG fights now become easy" and "previously easy mook fights now become hard", since that would imply the overall challenge hasn't changed and therefore that you don't really need to modify published APs.
While the overall challenge might not change on average, the extremes disappear*: When the easy and hard fights disappear, fun loses out. So you do need to modify published APs.
*) mostly; there might be previously hard fights against lots of mooks that now become impossible
So, the game breaking issue is basically what it was in 5e (of which I played since 2015-2016 right up until the release of PF 2e, so this comes from extensive personal experiences with the system) is that the HP and stuff isn't really enough to make the solo boss a threat unless the level gap is absolutely massive, or you use legendary actions + resistances to emulate the differences provided by level in systems with +level, i.e. spells and other control effects are generally less likely to take hold, the boss can do way more damage.
Level 9-11 parties should not be able to take down Solo Balors, but in 5e, with a reasonably optimized party, thats not even really much of a challenge. Its theoretically possible to balance a bounded accuracy, no +level experience such that it doesn't have that flaw, but 5e doesn't quite have the numbers tuned for that, and it does create a scenario where solo's have to be specially crafted with a band aid fix to their math.