Longer answer
We have, at least until December 2016, not gotten to know any free modeling tool that can really compete with commercial tools with respect to robustness, usability, feature completeness and availability-of-know-how.
Before you go for any free modeling tool, please make reasonably sure that your candidate fulfills the basic requirements we explained in Question F-2, concerning teamwork, robustness, versioning and artifact generation.
Actually, compliance to any specific UML standard is very often overrated.