Steps to reproduce:
Get a blah.ufraw file created with a pre-lensfun enabled version of UFRaw, i. g. 0.15. This will have no <Lensfun></Lensfun> tag whatsoever.
Run an UFRaw version with lensfun enabled (0.19.2-2) using: ufraw-batch --conf=blah.ufraw *.nef.
Result: Lensfun corrections will be used. This can cause trouble since depending on the crop size black borders will be visible.
Conclusion: Recent versions of UFRaw (containing Lensfun) should treat configuration files from legacy versions of UFRaw as if the user explicitly decided to deactivate Lensfun.