So I was having some issues just like you guys. I had a Geometry Collection in a Blueprint and tried to use Set Rest Collection at runtime , from a table with all the GC, and it kept crashing the engine. So I changed the strategy and first spawned a GC, without a defaut value, and then changing the rest collection. It solved the crashing problem, but then I was left with another issue, that physics weren’t enabled even if I enabled it when spawning the new GC (I would spawn a BP with the master field to simulate the fracture, but the field was spawning and having no effect whatsoever). So I googled it and found two solutions that worked (dunno which one actually resolved it): the first one was adding the spawed field to the GC’s Initialization Fields, and the other was to spawn the GC but with simulate physics disabled, and then enabling it in the next execution node. Hope it helps.
Sources: Spawn and set GeometryCollection initialization fields at runtime
https://www.reddit.com/r/unrealengine/comments/1ael6bu/need_help_with_an_issue_with_adding_geometry/