It is when compared to Firefox. Safari and now Google claim that it's for security, as the newer interfaces expose way less attack surface for malicious add-ons. I think that it's true that it does improve security, although I'm not sure that I believe Google when they say they're not doing it to stop people from blocking their main revenue source.
It cannot do dynamic filtering [1]. Another big one I forgot about is no CNAME uncloaking. Don't get me wrong, uBOL is still a great add on, it's just not possible to do everything you could with the manifest v2 version.
uBO Lite is exactly MV3, it has most of MV3 limitations plus its own limitations comparing to Chromium. Safari just auto converts MV3 codes to Safari's own blocking engines.