I have 7 years of experience with UE4… and I am not even trying to make a “multiplayer”.
It is not just hard to develop in the engine, it is also a problem to latter find a good server, some are way too expessive, and on top of all that… it is even harder to make all work with the specific store, even the stores themselves (talking from experience) tell you not to do it until you actually have a team of people monitorin sales all day long… It can become a target for hackers, etc.
The “use the marketplace” tip, I wouldn’t go for it… the marketplace is filled with lots of assets not even well made, or over-made so hard that your FPS drops to 8FPS by just loading those assets and projects, and if you try to use a plugin to make the multiplayer work, guess what, unless you have a massive C++ experience, what are you going to do when the developer of that plugin stops updating it (which happens a lot), what happens if it does works locally (which happens a lot) but after packaging and publishing it in the store, it just doesn’t work at all… now your neck is pending on someone else to respond your emails and fix that plugin in 3 months…LOL
Multiplayers are for bigger teams, and small indie studios (15 to 30 people) with experience in that field… or of course bigger studios.