The rear lens assembly of a well-designed clip-on will have some maximum amount of angular deviation for which it will have the consistent POI shift that you describe. That is the angular tolerance for that clip on. When you add a wonky handguard it may exceed that tolerance if it’s bad enough, or get close to it such that a wonky mount can take it beyond that tolerance. It’s a little silly to say “why worry about the mount and not the handguard?” One is a lot easier to change than the other, but the frank answer is you do have to worry about both because they both can be sources of angular deviation. Thankfully most handguards will be within tolerance, as will most mounts, but if you get a 95th percentile rail and a 97th percentile mount you might have issues. There’s a reason the manuals for every military clip-on I own say “mount it as close to straight and optical centerline as possible” and not “eh, fuck it, if you can see through it the results will always be perfect.”
