This Weapon Hurts High Level PCs
When characters are powerful you find it ever more difficult to challenge them.
If your milieu has civilized or wilderness areas, it’s an even tougher challenge.
In a city, for example, guards, merchants, and nobles will often no longer individually pose a physical threat. It would be unbelievable if every guard was suddenly 10th level or had 300 points.
Dungeons scale well. Monsters get weirder and more powerful each dungeon and you can make sense of that without too much story juggling.
But in other environments you need to change up gameplay.
The key boon you get from GMing factions is this:
At any given point in the game, you can bring a large faction danger to bear against the PCs.
While faction members remain the same power level as when PCs were low- and mid-level, you can pool faction resources and point that spear at the PCs.
It’s unrealistic if all members of the thieves guild became 10th level just to match the characters. But it is wonderful story grist to have faction leadership declare the party as priority enemies and buy one-off magic items powerful enough to seriously threaten the characters.
Perhaps the guild also buys some assassin contracts. Or they rob the wizard’s guild in an epic heist and frame the PCs. Or they bribe the bishop so the church announces an inquisition.
On an individual basis, faction members are weak and will pose little threat to characters.
But wield the monetary, political, and social resources of a faction and you can make even high level character feel low again.