Liberty isn't as inferior to tradition as it seems. It has policies that provide improve stability, and IMO is almost in every way superior then tradition accept for one aspect, happiness. Although liberty is great, I would advise picking up the pyramids so you can try and stay on top of the happiness issue you'll have.
Oh I meant more in relation to those Civs. Rome for example goes into negative GPT if you go gonzo with his UA unless you offset with a great variety of lux.
For Celts, Piety seems a better opener. If I try Liberty I try to be aggressive with Landgrabbing and for Rome that seems to get a bit too severe and with the Celts it just doesn't need to happen.
For Civs like Poland, Attila etc Liberty is downright sexy but replace them with Rome and for some reason I get weaker outcomes.