I notice in street and some R versions that people are drilling holes in the frame to mount thing with. Like 3/8" to 1/2" for seat belt/harness mounts and around 3/8" for nut-serts.
Isn't it weakening the frame significantly to drill large diameter holes in it?

Yes, I know it depends on multiple factors, like where, what plane and whether it's a major or minor structural member. Then, I guess number of holes in a single member can add up. I plan on using every opportunity to add brackets, via welding, rather than drill into the frame anywhere, but I'm building a race car. I realize this is problematic for those with painted frames and no real fabrication/welding capability.
How do you know what kind of overbuilding is done by FFR, that allows for big holes?
Is is SOP and nobody ends up with cracked frames?
Is there any rule of thumb for an FFR kit car?
I see the main frame, including roll cage is made of 1020 and I'm assuming the smaller square tubing is "mild carbon steel tubing" (whatever that is...).