Two of the three SSDs I bought for the buildbot hardware in 2016 have failed this year, so completely that the data on them could not be recovered. The third is well past its life expectancy and will undoubtedly fail soon too.
I bought one replacement SSD of a newer model to test it out. It's working fine. (The 10.7, 10.10 and 10.13 buildbot workers are on this disk.) I will buy two more SSDs soon.
Of the 7 buildbot virtual machines that went offline as a result of the SSD failures, 6 have already been brought back online (some on the new SSD, some temporarily on hard disks); the 10.12 builder is the last one to get back online (until the third SSD fails, at which point we'll have 3 more to deal with, but I've taken recent complete backups of them so restoring them should be simple).