First off, yes I am biased as I just died to one of these things.
The problem as I see it is there are enemies that spawn not all that far from the hub (not the outskirts, but nowhere exotic either) that have both stealth, and can disable you before you even see them via a melee attack, essentially killing you before you have a chance to make a decision that isn't "don't leave the hub". In my example I was fully buffed, and died in a single round (so not even being in a group would have saved me).
I'm all for making the road very dangerous and difficult to navigate, but this doesn't feel at all like high difficulty, its more of a lottery of "will I cross paths with instant invisible death or not". There's no reward or punishment for my decision making, its just a roll of a dice if I make a saving throw or not. Any other enemy, no matter how difficult, at least gives you the option to run away upon seeing it!
So, not to sound too cynical about it, but these particular enemies might need a second look.
These enemies are working as intended.
It is difficult to travel the Ash alone.
It's difficult, but you can also negate a lot of these dangers with listen or spot checks, numbers or extra defenses if you take such risks.