I haven't used the LLNL port myself, but I would be shocked if it were not rock-solid. First, ZFS (unlike, say, DTrace) has reasonably limited dependencies on broader system implementation; you don't have to port other subsystems to get ZFS working. Second, even where it does have external dependencies, it can operate remarkably well when they're not functioning: because of its indirect checksums, ZFS can operate correctly (or at least, non-fatally) in the presence of nearly byzantine behavior from the I/O subsystem. Third, of the ZFS issues I've seen and helped debug over the years (and my data has been on ZFS as long as just about anyone's), none have manifested themselves as data corruption or (in the absence of physical failure that exceeded the redundancy of the pool) data loss. Finally, of these issues over the years, virtually all were fixed inside of ZFS itself -- there was no platform specificity to either the problem or the fix. (The exceptions being platform-level I/O issues that resulted in pathological performance -- but it's hard to call those ZFS issues.)

tl;dr: absent glaring port issues, ZFS on Linux is or should be at maturity of ZFS itself -- which is to say, very mature.