Besides my belief that these reliability features may lead to system software of poorer quality because the software writers will simply rely on the restart, I have also seen too many bugs with the potential to render the system unusable especially thanks to the restarting.
In my case, these were all bugs in the start-up phase of the fault management daemon (fmd), which I happen to be sustaining as part of my job. You can find similar reports here by searching for "fmd full". The problem with fmd is that when it starts, it tries to process a backlog of unprocessed events, which is a non-trivial task. As with any complex tasks, there inevitably are bugs. So when fmd hits a bug in this phase, it dumps core and SMF restarts it. Then fmd starts to process the unprocessed events again and it dumps core again. This goes on and on until the core files fill up the /var file system and the customer calls Sun.