The ±10 crit mechanic seems to be what makes encounter scaling work. Higher level things do considerably more damage, but as you approach parity, the average damage levels out. I don’t think the system would work if one just eliminated it or tried to reduce its severity. However, it can be mitigated somewhat by using Proficiency Without Level. The scaling is still there, but it tracks much more slowly with creature level.
I’m not sure how badly it would break APs to run them as written modulo using Proficiency Without Level. Harder encounters would definitely be easier, but the easier ones could potentially a bit nastier. It would be interesting to see how the threat levels came out, but that’s probably a non-trivial amount of work.
Well, since we're having these discussions we must first ask ourselves what it means when an AP is "broken". If you mean that the unrelentingly fiendish difficulty disappears, then yes, it's "broken". Unless you instead consider that precisely the opposite of broken!
But generally, the fact easy encounters become slightly harder can't compensate for hard encounters becoming slightly easier.
Only the encounters that stretch the heroes to their limits matter. Everything else is just varying amounts of trivial.
Look at the XP awards handed out by PWL. You should see that you would need to regularly confront the heroes with monsters 5 or 6 levels higher than the PCs unless you want the task of getting the loot to become easier. That the monsters of lower level than the PCs become slightly harder should not be a great issue for the PCs, except in the (very rare) case when you have loads and loads of originally trivial monsters that now aren't as trivial.
For instance, you could previously have a fight against eight level -4 creatures. This fight would likely become non-trivial in a PWL party without good area damage spells.
But as I said, encounters going outside of the mold are rather uncommon. The vast majority of encounters feature 1-4 creatures, unless the monsters are higher level than you are, when there are rarely more than 2 creatures.
tl;dr: In my evaluation PWL is not challenge neutral. It will make existing adventures easier, since there are fewer challenge spikes.
Of course, if you create encounters of your own where you know you will be using PWL, this is of zero concern.