What's the catch?

I'm looking to get new phones, and was completely disappointed in Black Friday / Cyber Monday.

We have a 4-line account on Sprint, own our phones outright. Three Android users and one iPhone user.

From reading this offer, it looks like I could get 4 free(ish) Pixel 3XL's, port my four lines, and only be locked into Fi for four months. My iPhone user might hate it for a while, but it looks like I could ESN-swap her back to iPhone at some point. (It's a compatible phone, albeit in "beta" according to Project Fi.)

I might need to practice a little better data discipline to keep my monthly fee at my current level, but that's doable. (Right now I pay no attention to data, even though we have great wifi and home and office and could configure phones to use fairly little on the cell network.)

If I'm not liking it after 4 months, I port to someone else's BYOD plan.

Is there a catch I'm missing? Burning through WN GCs would be pretty easy for me to do.