A recent build of the MK includes Facter 2.0.1, and that is definitely incompatible with the code as written – we grabbed the version in a way that wasn't technically "API", so that broke.
I understand from the public docs that Facter 2.0.1 also brings structured facts to the table, so we have potentially greater incompatibilities as yet uncovered.
We also shipped in PE 3.2 with Facter < 2.0.0, so this is a source of two potential incompatibilities:
One, users who rebuild an ISO image to include a custom fact, or custom binaries, may no longer be able to do so with PE 3.3. This is a reasonable common question on the mailing list, so we can infer it is a reasonably common behaviour.
Two, if Puppet ships with Facter 2.0.1, and the MK has Facter < 2.0.0, facts written to take advantage of new DSL features in Puppet will not be compatible with the Razor MK – and given we are making it easier to extend at runtime, without having to do a rebuild, we can expect more users to take advantage of the (frequently requested) feature.