Well, this works. But you can attach only pure mesh components. If you want to attach an actor, then it is problem. I would like to have Car BP (just car) and Bed BP (contains the bed and tailgate connected with a constraint) and attach Bed BP to the Car BP (or attach different Bed BP etc.).
If I doing everything correctly, then I think that is a bug in UE and they will not fix it (they does not care about vehicles). Your solution will be maybe usable (still don’t know what will happen with animations and other things) but it will complicate everything and I will have to save values, settings and sounds from the bed to the cars which is mess. Maybe somewhat to connect it with scene/actor component, if it will work? But this is probably only one solution when I want to have separated car and bed.
Another one is to have car together with bed+tailgate. There I cannot turn off physic simulation to kinematic (for locking tailgate), it still simulating. I can only enable all constrain limits but still is not locked on the place as rock (moving slightly when crash).
Maybe I have to something un/tick something in the actor to work, but dunno 