ID/creationists are the only one who thinks that UCA has a “lack of a model.” And they’re absolutely wrong, because it is a testable theory that has been confirmed time and time again. See here for just a few tests of common ancestry that have been performed.
How the changes occurred is irrelevant to UCA, because UCA is confirmed by the pattern of similarities and differences, not the similarities and differences themselves. The evidence for UCA is compatible with both God’s guidance and unguided evolution, which is why even some IDers (such as Behe) also accept common ancestry.
I’m open to challenges to the theory of UCA, but there aren’t any serious challenges as far as I’m aware.