Isn't BSD Sed a subset of GNU Sed? I can't think of any outright incompatibilities that would make a BSD Sed script invalid for GNU Sed.
Their handling of the in-place flag is different enough to cause irritation if you forget which one you're invoking.
I'd like to change it in FreeBSD, but haven't really (yet) come up with a clever way to make it work without breaking years of scripts that may be using -i.
Are you a maintainer of sed on FreeBSD?
Not sure we have anyone that could really call themselves a maintainer for sed, but I am a FreeBSD committer that has put thought into this problem in the past.