From the user and developer point of view, the only major drawback would be applications. Devs porting over apps to linux and users not having some of their favourite apps available.
It's becoming clear that Apple is not paying enough attention to the under the hood details of an OS and the security situation. And maintaining the OS is a huge investment from Apple. So it only feels natural to base Mac OS on linux and, as I said earlier, focus on the distinguishing features.
Technically though, it would still be a huge challenge as there are lots of factors to consider,but in the long term everyone wins Apple,linux though maybe not Microsoft.
I have been thinking about this idea of Apple using linux instead of its proprietary Mac OS for a long time, and have considered the transition only at a very high level. I would like to hear what the HN community thinks of this idea.
Edit: So instead of forking a version of linux and developing it independantly like what Apple did previously, a better model might be where Apple periodically forks from upstream projects and makes its modifications. Something like what Linux mint does with Ubuntu. So this way, they continue to get the latest and greatest developments from upstream projects without investing additional resources into maintaining their forked OS(this is currently what is happening with Mac OS).