MAMEUI has been considered "bad" by the MAME developers for at least 10 years. The modifications to MAME needed to make MAMEUI are incredibly invasive and damaging. Yes, damaging. MAMEUI has routinely had bugs above and beyond baseline MAME.
That said, MAMEUI was not "targeted for destruction" like some seem to think. The aim of getting the code GPL was to add to the ease of maintenance for the future-- MAME now has the ability to assmililate other GPL-compatible cores and other code, and for other GPL authors to use MAME code.
You're not stuck with the embedded UI. There are dozens upon dozens of extremely high quality front ends that don't require any invasive surgery and thus will continue to be easily maintained and won't care one bit about whether MAME is under the old MAME license or the new GPL one.
MAME as a whole is not about "playing old games". It's about preservation, and moving it to GPL is one step closer to getting it into museums and other places that would really love to use it for such.
As for liability, I'm no lawyer but I can tell you that compiling a new MAMEUI without getting the license worked out would put you at risk of being sued by any single person who has contributed to the MAMEUI code over the past 20 years and has an objection to their code being GPL.
I don't see this being a huge problem, at least partially because it's being handled appropriately. Rather than screaming and crying, at least one responsible MAMEUI dev is already approaching as many code authors as possible to get things licensed properly and likely will have this resolved before a new MAME release comes out.