I think stanron (and I) were just trying to keep things simple, ie. AEIOU rather than suggesting other letters may not be used as vowels.

I think my "crwth" is the only one entered here that doesn't use the "y" but even there, the Wiki article I linked to says:

Like several other English loanwords from Welsh, the name is one of the few words in the English language in which the letter W is used as a vowel.

cwm is the only other with the W I'm coming up with at the moment and that's 2 letters short...