I know but LC475 works with switchable adapter!
So there must be something else...
"but apples and oranges!"
You're comparing two entirely different machine architectures. If the LC475 works at 512x384 on an LCD monitor, I would gather that the horizontal frequency is higher than 24 kHz.
Has anyone tried one of these?
I have, they're junk. There's a whole market of those conversion boards and an infinite number of clones of clones of clones. You never know what you're going to get, you just know it will be trouble. The stock firmware on those units leaves A LOT to be desired. The arcade community has done a bunch of hacks to make that conversion board work properly, because many of the things the people that sell those boards claim it does, it in fact does not do, or even close to.
The thing I bought one of those boards for was RGB to VGA, it didn't work. You would get a picture, but after about 10-15 seconds, the output would get unstable and desync repeatedly. No amount of bodges or workarounds would get it to work. In the end, I found a cheaper method of replacing the RGB encoder chip with a slightly different model that had Y/C out to make an S-Video circuit. 95% of the quality at 25% of the cost.