Is it possible to branch a program so that it send either an email or creates a CTA based on NPS score (and the tier of client)?

Yes.  In a Conditional Wait you can have multiple conditions such as Detractor AND Enterprise.  But you can only have two branches out of a single Conditional Wait so your choices would be Detractor  AND Enterprise on one path and everyone else on the other.  If you want something like: If Promotor send a Thank You but if they are a Detractor or Passive send a 'Very sorry' email for small clients or create a CTA for large clients, you could do something like this: in the NPS Program model after the "Responded" fork have a Conditional Wait that checks the NPS response (you can access any survey answer from this Conditional Wait step) and if it is Promoter fork off to a Thank You email.  Otherwise go ahead to a second Conditional Wait (I think you will have to put a short Wait Timer in between) that checks a participant field to decide whether to go ahead to a 'Very Sorry' email or to a Create CTA step.

Thanks for the reply. I have similar experience using conditional wait steps on some marketing automation platforms! Sounds like it's similar where it's almost a ladder down to make sure every path is covered.  Glad it can be done!