Hi,
you have to weight how much budget you have against how likely your game is going to get hacked.
yes server side is in theory more secure, but what prevents somehow to hack your server instead of your client side code, if you run your own server without full expertize and experience, It's !guaranteed! you'll be hacked big time and have your server used as a host for streaming bad content and you could end up in real trouble with very real consequences.
So yes, if you are an expert and have budget to hire proper server side coders and webmasters, it's better, but it's unlikely, that's why Photon is providing this "serverless" solution where you can do it all within the client and forget about the server side nightmares all hosts have. It has some limitations but the benefits for indies and small studios totally outweights the cons.
calculations on clients is marginal actually, if your game gets very successful, you'll actually end up with problem for your servers spec that won' be able to handle traffic and volume of processes, while on your client side code will not be stressed more because you have thousands of Players, yes? So the more your game will become successful the more it will cost you on the server side while it won't affect at all your client side, 2 players or 1 millions player won't change anything per device.
so, Photon server exists and it's free actually, but you end with having to pay for the servers that will run your photon servers!!! it's not for indies and beginners, it's for advanced experts with a proper budget.
Bye,
Jean