Power of Themes

Im a newb so I have some questions about themes. It seems to me that alot of themes are more powerful then their orignal or original is more powerful then the themes.
For example, Last of the tuatha, seems more powerful to me then regular man. You lose avalon witchs/warden/knight of avalon but gain three powerful mage preists, who are all stealthy, compared to witchs of avalon who are not, and you gain two great blessable warriors. So in that case, why not always play Last of the tuatha.
Tell me if im wrong but it seems to me:
Reg Abysia>Blood of Humans
Reg Pyth>Serpent Cult
Masiama>desert tombs/reg ctiss.
Reg Ulm>Black forest/iron faith
Reg Arco>Golden Era
Reg Caelum>Return of the Raptors