Hey all. I have been r&ding the new rbd car workflows in Houdini and have been stuck trying to figure out why my vehicle collapses even though there are constraints connecting everything.
As a quick breakdown. I have a mustang model that I cleaned up with new and organized naming conventions and groups. I used a rbdcarrig sop to pack the geo and set up the body and wheels while also giving it a speed value. I used the new rbdcarfracture sop to set up the constraints for the different components of the vehicle. When attaching a rbdbulletsolver sop, even with default settings, it falls apart during simulation.
Any advice to help me troubleshoot? I assume many other artists struggle with this setup as well. Please let me know if you have any questions concerning info I may have left out or need any clarification.

It's hard to tell without seeing the hip file unfortunately. If the car pieces fall apart right from the get-go, it means one of 3 things:

1. the constraints anchor names point to pieces that don't exist
   
2. the constraints break and disappear immediately upon impact with the ground
   
3. there are no constraints between the pieces that break apart
   

For 1) a hip file would really help to see what went wrong with the constraint creation.
For 2) the vehicle mass is too high or the constraint strengths are for too low and you have constraint breaking thresholds enabled for all constraints (including the soft constraints for metal parts of the car which shouldn't break) so the glue constraints break immediately, switching to the next constraints (soft with plasticity enabled) which then get deleted by the constraint breaking threshold as they probably stretch beyond the default distance threshold.
For 3) Make sure you have neighbor constraints enabled between the sections - you only want to disable those when you want to create your own constraints, i.e for a door where you want to position the hinges precisely, or the hood, or the boot, etc...

If it's none of those, it'd be really helpful to see a file. If you can't share it publicly, file a bug and upload it there so I can take a look.