Great question — answering as the UphillTech team:
I think bhyve is your first resort, not your last. If you jail your build environment, then you're testing your product against a 6.3 world and libraries, with an 11.x kernel. Your customers however, will be running a 6.3 world/libs and a 6.3 kernel. Differences between the 6.3 and 11.x kernels can be expected to increase over time, I would imagine. This could conceivably lead to situations such as one in which your test jail's 11.x kernel Does The Right Thing, but your customer's 6.3 kernel doesn't, and therefore, you can't reproduce your customer's problem report. IMO, bhyve is the best tool for this job.
Full answer by Jim L.: https://superuser.stackexchange.com/a/1308864 (CC BY-SA 3.0).
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