I'm not sure what you mean by a 'technical' reason. There's nothing in the software to keep you from using it...
This is similar to the discussion of a couple/few years ago with the European Aristocrats project (before I joined Wikitree) with regard to prefixes or prepositions in the LNAB field. There were respected people on both sides of the discussion. They decided to not use them in the LNAB field so that people would more easily find them when searching.
It seems it would be good to be consistent with this for the Cymru project also. My other concern is that people creating new ones might improperly use ap or ab and thus create a duplicate. I find it somewhat confusing myself! I located this discussion on soc.genealogy.medieval from 20 years ago: http://archiver.rootsweb.ancestry.com/th/read/GEN-MEDIEVAL/1995-09/0811805168
The person noted: "The difference between "ap" and "ab" is the sound which follows it. We do the same in the English language with "a bicycle" and "an orange". The spelling and pronunciation of the indefinite article changes depending on whether it preceeds a vowel sound or a consonant sound.
"In Welsh, "ap" comes before a consonant sound, thus "Einion ap Llywelyn", "Jenkin ap Gruffudd", "Llywarch ap Bran". "Ab", on the other hand, comes before a vowel sound, thus "Maredudd ab Einion", "Huw ab Owain".
"The reason I said "vowel sound" and not "vowel" is that in certain circumstances the initial letter "I" in Welsh can sound like a consonant. Thus "Rhys ap Iorwerth" and "Dafydd ap Ieuan". Kind of like in English where an initial "U" can sound like a consonant. We say "a university", not "an university".
"As you would expect, not everybody knows enough Welsh to handle patronyms properly, so you'll find lots of variations in the forms, even among people who write on Welsh genealogies."
Darlene - Co-Leader, European Aristocrats Project