There is zero lock-in with the files being saved - you can export from the app in HEIC/PNG/JPEG as well as PSD and spatial/SBS 3D. The format the website calls the 'Photosynthesis' file is an HEIC so even if you export that you can still access the data outside of the app.

Unlimited exports of the composite images are part of the subscription (with an option to automatically export to Photos.app and/or a folder of your choosing), but the free tier still offers unlimited saving of the constituent images e.g. the wide & tele shots, just not the composite.