It's just a topic conveniently segregated when viewed under the racial lense (shitstirrer suka ini), when it's just an academic certification much like IGCSE.
Almost all arguments against UEC should be applicable to IGCSE, yet somehow that's not the case.

There is however a real, distinct disadvantage for local university admission / work position due to not focusing on BM or English which is the more widely spoken language in Malaysia.

Here, have a post by Barisan Nasional, the OG fanner of flames:

Like I said, it's just another form of academic cert., but western punya cert. got the advantage of using english as main medium which is still way more widely used in Malaysia.
IGCSE students if not mistaken are required to take a separate BM/Sejarah paper if they want to join local intakes,
but (some) UEC proposal want to make do without either and terus be 100% equal with SPM certification.

The rest semua racial lense viewing.

Same with Jawi what, just an ancient south-east asia writing system yet bila ministry want to introduce kena fearmonger trying to proselytize nons.
What is actually Sejarah-related subject become Pendidikan Islam-related from all the hoo-haa.

My take is rich cina folks who send their kids to UEC is also big part of chinese party fund contributors,
hence making the party having to fight for that interests, cause like you guys said most average chinese also dunno party fighting for whose interest exactly

On the recognition part best guess is they want backup in case their UEC-passed kids actually tak laku under competition so would be nice to have backup pathway locally.