Resolve understands fades, dissolves, and clip speed.
Things like motion effects, filters, are ignored by resolve but kept in the timeline when brought back to FCP. So if you don’t do any repositioning inside Resolve, your project will go back to FCP perfectly.
Any titles you had will be kept, although show up as offline clips in Resolve.
Actual speed ramps with variable speed are not supported.
All in all the roundtrip is pretty good!