Not necessarily. Example: for months, everyone thought it was the Saab's fault that X-Plane was crashing when run with xEnviro, because it wasn't crashing with any other add on. We thought so, at first, too. To the point that Jim and Ben went through the Gizmo code and the Saab code to see if anything was amiss.
Andrej (xEnviro developer) finally decided to look into it, at my urging, and with several other people willing to run some tests, found it WAS xEnviro at fault. The problem was found and the crash happened whenever xEnviro would pull weather from the weather server. It was fixed for their next update.
Add ons are made to run with vanilla X-Plane. If a crash happens, we'll look into it, but we can only do so with a log file and with the help of the user by eliminating some plugins and seeing if the problem disappears. Then we can take it further depending which add on is to blame.