Unless you have an ownership interest in the startup or are a founder, I don't see any risk with getting a TN through a startup and even changing TN companies every year.
What about if you are a founder with a minority stake? Would you recommend some other form of visa?
Less of an issue but CBP and USCIS will oftentimes deny TN petitions where the applicant has any ownership interest even though this is legally indefensible.