Well, folks like Pressburg have shown that when they bear down on the gun, they can cause 6 to 8 MOA of deflection. Pressburg has also hypothesized that handguard flex is part of what's driving some folks that he knows and respects that do significant amount of work under NODs to move their MFALs back to as close to the receiver as possible, despite the many downsides of doing so. There is also the fact that some modern guns still have significant handguard flex issues, such as the MCX, particularly the newer Virtus models, enough that folks have to come up with mounts specific for a laser to tie it to the receiver instead of the handguard:
Not 100% sure of why GBRS created the mount they did, but it's been hypothesized that this would not only allow a better weight distribution and grip (one of the original reasons folks mounted their lasers to the rear near the receiver for more daytime oriented units), but also to completely circumvent the handguard flex issue (which was also a reason folks back in the day would mount near the receiver, even when NODs oriented, when there wasn't really many choices for low flex handguards):
https://i.imgur.com/Rnrg9jP.png
....