What I've gleaned from interviews is that the simulation part of the game including the netcode will be completely custom. The way I understand it, they're effectively only using Unreal Engine for rendering, sound, UI, cutscenes and so on, but all the gameplay logic is their code. Customizing an off-the-shelf engine to that level has to be highly nontrivial, and probably out of reach for smaller projects.
This is the only fear I have with 100% DIY - If I want to go much beyond 90s graphics, I will need to do a lot of extra work (aka "draw the rest of the owl") to even begin talking to artists.
That said, there are ways to build games that do not require elaborate art tool chains. AAA visuals are the antithesis of my target. I am perfectly happy trying to do art in code until the wheels pop off the bus and I have to retool for an external art team.
How many completed games have you shipped with your custom netcode and engine?
Networking is very fundamental to some types of games and the expected player experience. Grabbing a generic "this is multiplayer :D" box off the shelf and cramming it into your concept will likely fail miserably if you are seeking a high-quality outcome.
Certain flavors of net code cut extremely deeply into the game engine concerns. Rollback net code is a pretty big dragon in that it suggests your simulation must remain deterministic at all times. Ideally, if you are planning time traveling style net code, you would first have this problem cooking before you send a single byte across the wire.