I had never encountered a segfault while streaming in earlier versions. The previous version I was using was 1.6.5. I got 1.6.7 to segfault first time I tried it. I was using XBMC as the client. I tried jumping forward and back between titles. This usually causes XBMC fits. XBMC did hang. After I killed it, I noticed that makemkvcon had crashed with a segfault.
There were no abnormal messages on stdout or stderr. I do have a core file if that would be of help. This was on 64-bit Gentoo Linux.
So far I would rank this more as an annoyance than a show-stopper. Until now once the streaming had started, makemkvcon was pretty solid even when clients were crashing.
BTW: a FUSE interface sounds awesome.