Not so much wrong as in why? You have 4 less levels to play around with. With 10 levels of Ur-Priest+PrC progression, you only have 6 other character levels to play around with.
X6+UP2+MT8 gives you Arcane 7ths, and UP 9th's, but at that level of game breaking there's no need to be a Flind Gnoll, just polymorph into one, and a better build is probably X6/UP2/MT7/Contemplative 1 for the Domain, and the same issue resolves, you can easily get access to Dual 9ths and Polymorph or Shapechange into the form you desire by that stage.
Flinds bonuses equate to something spells can easily cover;
Stat Ups - Bulls Strength etc, later polymorph
Darkvision - Darkvision spell
Natural Armour Bonus - Alter Self
Weapon Familiarity - Heroics for EWP.
Ur Priest won't have access to these, thanks to lack of domains and those mostly being Arcane, but if you have any Arcane Spellcasting progression into Ur Priest,
1) You're wasting your abilities that you took Flind Gnoll for as you proceed to cast spells with reduced BAB and HP early on
2) You can obviate the need for Flind Gnoll in later levels.
