This, in itself, is the reason I shy away from home builds. I’m sure as hell not a ‘smith so I would not know a s**t build vs a professional one.To some degree, they are a gunsmith. With all the various manufacturers of parts, not all built to the same specs, it's not really *just* an assembly job like Lego or an Erector Set. But sure, you're not doing stuff like lapping sears, milling receivers and glass-bedding barrels.
To assemble an accurate and reliable AR15 requires some understanding of what needs to be properly 'in spec' and installed, but you're right, many of the skills of a traditional 'full spectrum' gunsmith aren't required, such as machining.