But then, it seems there are quite a few un+latin (unreal, unbalanced, unadulterated, uncertain etc), even if I can't think of in+Germanic.
https://www.etymonline.com/word/unbalanced#etymonline_v_2490...
Like most things in linguistics, the 'rules' are more a rule of thumb than the mathimatical sort.
Germanic pre/post fixes seem like they stick to pre-inkhorn roots better.
https://en.wikipedia.org/wiki/Inkhorn_term
But then, it seems there are quite a few un+latin (unreal, unbalanced, unadulterated, uncertain etc), even if I can't think of in+Germanic.