[Nemo]

There are some practices that make large assemblies behave better in Inventor. Make sure there aren't any warnings in your sketches so that constraints don't fail; constrain to work planes and axes when possible since you know those aren't going away; turn off adaptivity; don't apply redundant constraints. The way one chooses constraints in sketches and in assemblies is going to have a lot to do with how often the model does weird things or crashes. Also, if the model is built in a funny way and then a modification breaks some relationships or causes constraints to conflict, that's going to make the model act weird. I can think of some things make a model resistant to problems like that, and if anybody has a guide like that it would be a useful resource. The OP probably already has a good feel for that from using Creo.

Also, if you demote some parts and then make the new subassembly flexible, you can see problems in some cases, because certain constraints will get broken depending on how you had it constrained before demoting the parts. That is easy enough to fix by going into the subassembly and adding new constraints.