You cannot self-host Photon, so you’re basically locked-in with the vendor. While this may not be a big issue now, I do remember the story of Game Spy Arcade. It was well established network solution for games, but it got purchased by 3rd party some years ago and the new owners requested tens of thousand dollars monthly upkeep from existing games (most of them pretty old) or they will shutdown the servers. Actually that’s the reason you cannot play online multiplayer with many old games from GOG catalogue, unless you use VPN and the developers actually allowed LAN plays.
This is why we here stick to Nakama service. It is not as advanced as PUN (although their match-making is somewhat near), but its Open-Source Software (meaning you can always patch yourself if some feature is missing) and you can host it on your own infrastructure (or you can pay for the hosting in the cloud by Nakama, which is basically what Photon is offering).