I looked into building an adu a few years back, and I can backup most of what you said. At least in California, in an incorporated city.

We worked out that it would cost us $30k in fees, and required spending before we could even dig the footers. Things like a soil test, inspections, variance’s etc.

You can always build an ADU, but it’s a rich mans game. And not really accessible to most people. We calculated our unit to cost roughly $80-90k, and would be almost 2x $/sqft of our home. A remodel made more financial sense, but the above problems still persist.