Most of the recent crater runs I looked at have a ton of crates that "regressed", but there's no build failure to be seen in the logfile. For instance, in https://crater-reports.s3.amazonaws.com/pr-129199/index.html, all the items I looked at in "build failed (unknown)" fall in that category. Concrete examples:

• https://crater-reports.s3.amazonaws.com/pr-129199/try%232c70eb47850052381670935f807d41647a7dc992/gh/pba-pba-pba.blockchain-from-scratch/log.txt
• https://crater-reports.s3.amazonaws.com/pr-129199/try%232c70eb47850052381670935f807d41647a7dc992/reg/cryo_python-0.3.2/log.txt
• https://crater-reports.s3.amazonaws.com/pr-129199/try%232c70eb47850052381670935f807d41647a7dc992/reg/earendil_packet-0.5.8/log.txt
• https://crater-reports.s3.amazonaws.com/pr-129199/try%232c70eb47850052381670935f807d41647a7dc992/reg/waynest-0.0.9/log.txt

Also see here for some more examples of this.