I have done quite a bit of testing on how darktable and digikam play together. Like mentioned before, they use the same naming convention for xmp files and digikam can be configured to write metadata to xmp sidecars only. There are some subtle differences to how certain kinds of metadata is handled, e.g. color flags.
Yes, see https://redmine.darktable.org/issues/12304 for example. The only workaround I could find was to use darktable with a :memory: library (i.e. darktable --library :memory: . or change the config file). That way, starting darktable takes a bit longer, but when working with not-too-large folders it is acceptable for me.
For the other way around, changing metadata in darktable and having digikam reload it automatically, there are also some bugs (c.f. 397340 – Update image file timestamps if metadata is written to sidecar file [patch] 39833 – Kmail crashed after notification of new mail), which should be fixed in the upcoming digikam 6.0.0. 
In my opinion, the situation is still far from ideal, but when being careful it should be quite possible to use digikam for culling, organizing and managing descriptions/tags and darktable for editing.