World Creator became my weapon of choice after having tried every terrain tool available over the past few years. There is plenty to be said there, but I can touch on some of the subjects you’re interested in.
I suppose the best tile size depends on the total size of your terrain and the usual suspect such as target platform, visibility and gameplay. I’ve been using it for an 8x8km terrain, split up into 256 tiles for streaming. Each tile ended up with a heightmap/splatmap resolution of 128px, which is not very high but sufficient for my particular project. The resolution is directly related to the detail per meter you set up in WC, you can freely change this as it’s non-destructive. Though it’s not possible to control the resolution per tile.
As for the splatmaps, these are equal across all tiles. If texture X isn’t visible on tile Y, it’ll still take up a splatmap channel. Not ideal, but also not a trivial issue to circumvent.
I’m using 2018.2 and the standard pipeline, so I’m afraid I can’t offer any experience on those fronts. But seeing as it doesn’t use any graphical features, I believe it should work.
I feel like the sync tool is functionality, but not field tested and polished. One thing you’ll find is that the syncing operation first removes all terrains from your scene, then creates new ones (in the Assets root folder). This means any setup in terms of hierarchy, layers, scripts and references will be obliterated.
Running the syncing operation, then discarding the scenes changes won’t do any good, since the order of the tiles will be changed (I believe they’re sorted based on their maximum height). For me, I ended up creating a boatload of post processing operations to setup all terrain properly again after syncing (adding scripts, adding MicroSplat, registering to Vegetation Studio, etc). There are a few more quirks the sync tool has, but I shouldn’t be too negative, the fact that is exists is awesome!