This is just opinion,no legal basis:
If you are fixing your sister's handgun,I do not see the factor attracting attention,or the concern of the BATF.Low key,no problem
However,suppose you are a Cowboy Shooter and the folks in your club figure out you are a good hand at keeping their 6-guns,Win '97's and lever actions running.You come home from shoots with 2 or 5 guns each month,suddenly,you are not so low key.You get famous.
Suppose you do a fine job and have happy customers....and they mention it to the guy with the brick and mortar gunshop paying taxes and fees and filling out paperwork trying to make a living,and being inspected and audited by the BATF.This gunsmith might feel frustrated and complain.
You drill and tap for a scope base,and your happy customer has a negligent discharge resulting in injury/death,or loads 72 grains of Bullseye in his 300 win mag(don't do that,its a blow up the gun load)so now,there is an investigation because somebody is hurt,lawyers are involved,and you drilled and tapped it,or worse,put on a recoil pad!
The custody thing could get important,example,you get stolen from.Six firearms are missing.They have 5 owners,not you.You are explaining the stolen guns to the police.
You get some AR tools,build your own.And,you do one for a buddy.Your buddy's cousin.They shoot good!After a while the local constable and DA have heard a rumor some guy in his garage is putting hundreds,thousands,jillions of EBR's on the street.
IMO,the penalties are high,and a variety of politics can enter into the mix.I choose to be squeeky clean legal.I do my own work,but explain to folks who want me to build for them "Law requires an FFL and I do not have one,so I do not take in work'
As I said,just an opinion.