We are internally Building both (proprietary) Engine code and Assets with Bazel.
Now for a new Project we use Unreal.
For Bazel the general direction is to get (transparent) caching and remote execution available to everyone. See this spec. We are working to get this available for our internal Bazel builds.
For our Unreal project we have similar requests.
Having worked with the Linux port and UBT in the past, I did take a look at current UBT. The internal action architecture seems to be quite compatible with the Action Caching of Bazel. As such my question is whether, if someone would extend UBT to make use of Bazels Action Cache, would you accept that for inclusion into UBT?