What's In a Faucet. Can I Make My Own Faucet from Various Parts?

Flex lines go from valve (mixer or two separate valves) to the spout, more and more these days.
Roughs are not all "hard" brass these days.

In https://terrylove.com/forums/showthread.php?10876-What-s-in-a-faucet
I saw
"Rough-in valves are brand specific"
also
"....valves are EXTREMELY specific to brand and model. Any attempt to rig in or use "universal" handle adapters will lead to great sorrow."

I'm not sure I understood these comments.
To me, they make sense with the hard (brass) roughs, but not with valves connected using flex lines.
Is this right or mistaken?

In other words, can I connect a valve, and a spout, using flex lines?
And in doing so, am I taking huge risks or minimal risks?
My flex lines would be above deck in a shelf (similar to K-2440, K-2443, from Kohler).
My handle would be the one sold with the valve.

All I'm seeking to do is send mixed water to a new spout, through flex lines.
I could also solder copper or brass to connect it after I get the dimensions.

Thank you!

Those comments are MORE relevent with flex lines, since many of them use proprietary connections which you CANNOT interchange with other brands.

let your imagination run wild but remember that the spout can not be able to be submersed in the sink because that causes a backflow situation. I have seen faucets fabricated from all sorts of crazy things.

Not big hurdles. Threads on flex connectors may become a pain, but are not a deal killer.

Backflow preventors are common and easy to buy.
They are built in to the faucets with pullout sprays on hoses.