Yep, this is true. Currently typing this on an iPhone SE with Project Fi. For MMS, I highly recommend using Hangouts (particularly for voicemail). But otherwise, I use iMessages. I have been doing this for the past year, it’s been working smoothly.
One of the big features of ProjectFi is the worldwide roaming, and we can't get that on an iPhone from what I've seen. MMS via Hangouts is nice though.
