This makes bugs appear, and get addressed, and eventually gives you confidence that nothing nasty remains uncorrected, and it won't eat your data.
ZFS on Linux, due to the unfortunate licensing situation, is considerably less tested and thus scary, data-eating bugs are at least a bit more likely to exist.
Yes and no. Few people, even among the Linux crowd, are aware it exists or feel like trusting it with important data sets/jobs. Those few are the most likely to be equally at ease running FreeBSD or Illumos (or Solaris), where the damn thing is known to work really well.
In practice this means that ZoL may require more memory to be stable, or may be less stable depending on configuration if low in memory.
I've run ZFS on Solaris (by way of Nexenta) and Linux since 2008 or so. I haven't seen much reliability difference in practice. I've had fewer hiccups streaming video from Linux though.