I've just installed the Namco System 1 Multi and it is working great so far; I'm missing a SEEQ chip for now so I've got one ordered before trying the full list of games.
However here's a precautionary tale: If you remove the dipswitch block for whatever reason, make sure you reinstall it over all of the pins; the shroud around the pins on the multi allows you to install the dipswitches to the left or right by one pair of pins. When I did that, I got nothing but a black screen when I powered on. Thankfully all I did was try pressing the boards together before I spotted what I had done. I nearly ended up taking the boards apart, removing the multi and reinstalling all of the original chips to troubleshoot.
However here's a precautionary tale: If you remove the dipswitch block for whatever reason, make sure you reinstall it over all of the pins; the shroud around the pins on the multi allows you to install the dipswitches to the left or right by one pair of pins. When I did that, I got nothing but a black screen when I powered on. Thankfully all I did was try pressing the boards together before I spotted what I had done. I nearly ended up taking the boards apart, removing the multi and reinstalling all of the original chips to troubleshoot.

. I had actually confirmed everything was initially working with a few games and then removed the dipswitch block to see if I preferred having it on the extension cable. It was only when I put it directly back on the PCB that I got it wrong. So I went from a fully working multi to no games working, that was when I started checking the multi was still seated correctly, etc. when in reality there was nothing wrong with those connections, it was just the dipswitch block in slightly the wrong place.