Most of these Bard PRC seem to have one of 2 weaknesses:
1) They grant a non-stacking spell progression, like no spells at all (Arcana Archer, if you see it as a Bard PrC), or have their own progression.
2) They grant a stacking spell progression, that is not limited to Bard spellcasting.
The first one simply sucks, the second one usually benefits sorcerer/rogues or similar classes more than the bard itself. Full bard spellcasting isn´t that great compared to the alternatives...