ULX's use of motd isn't much more than a Server "opening text" - "send text to player" and then Client "got text" - "open using Source HTML viewer used by Gmod."
Very few problems I could see actually caused by ULX.
- If the html file is huge, ULX could take a while to send it.
- If your line is saturated (at 4 players), all other data takes long to send?
Maybe others, but thats all I can think of at the moment.
Now, problems caused by Gmod and it's use of Source HTML viewer...well, that is a whole other ball game.
Incorrect html, odd CSS, dhtml, heck... Source HTML itself is often just flaky.