The free user limitation is ok. If you get more users constantly playing your game you're probably earning enough already to cover the fee of the next tier, depending on how you well you monetize your game of course.
Photon supports all platforms. So far i have had no problem with either android ios, windows phone, and pc all can even play against each other in the same rooms. I'm targeting mobile and i think it's perfect for that purpose as your don't need the host/peer setup. I guess Mobile users wanna be able to just jump in and play at any time against random people, or create rooms to play with friends... very simple to setup. You can disconnect from a room and keep the room open until the last one leaves.
For me it was very comprehensible...
if player 1 moves... you send that to the photon server which then sends that movement to the other clients.
if you fire a shot. You can first send that to photon server then to all clients including yourself to have the shots somewhat more synced.
Most of the time the lag is minimal. I haven't measured any exact ping but on mobile, any lag will probably be caused by your mobile connection, not photon.
I'm not an expert but if you decides to play around with it, I'm happy to answer some questions.