Its not that its better built . The Fostech uses what is essentially a safety sear that is tripped off the closing bolt carrier to keep the hammer from over riding the bolt. Franklins do not and can run faster than the bolt carrier. Many guns have safety sears. SKS's, Winchester 100's etc. have safety sears. Configured a little differently safety sears are called a automatic sears and yes, a clever tinkerer could handily turn the Fostech into a drop in Full Auto Conversion device. Simple job.
And this Friends is why I have a lifetime ban from the High Road.
Hahaha! Man, I wish some of the old home brew and DIY gun forums were still live. I should have copied everything I could.
You are essentially talking about the rate reducer in the AK too. It's actually a safety feature but the ATF considers it bad as it is a required part in a FA AK. I mean, I could probably make one run without but it'll be more reliable with.
A couple pieces made with a grinder, a file and a drill and you can make a binary trigger for the AK too. It'll even work in the right selector positions unlike fostech's.
As far as the fostech and the BFS, the franklin is also capable of running faster than the fostech but as wired said, it can outrun the carrier and have hammer follow through which could simply make a dead trigger.