I haven’t yet tinkered with Source 2 so I can’t really speak for it, but if it uses the same networking model as Source 1 did, then pretty much everything networking related is a lot more convenient to use in Source. UE4’s networking model on the other hand is kind of twisted in the sense that all replication and RPCs have to work through some actors (entities) with a whole bunch of limitations involving the actor’s net owners. No point in diving into details, but it’s an inflexible mess that only caters to a very specific networking model, which is also harder and slower to develop with, and will make you rip out hairs whenever you’re trying to do something different the Epic guys didn’t intend for. You could go about redoing it at the cost of time and effort.
Additionally, from experience, C++ development in UE4 is hell, despite the rare few that claim differently - anyone who’s done a big project enough C++ project in UE4 will confirm this. 75% of the time it’ll feel like the framework is backwards and oddly constrained.
If I had to pick one engine over the other for a new first person shooter, it’d be Source 2, just because Valve has exceptional standards targeting a very specific goal (which is very much aligned towards first person shooters), while UE4 feels like a clumsy hack of modules stitched together.