As far as performance and feature set, probably not anymore (I would have answered differently 10 years ago, and if I am wrong today would love to be educated about it).
However, if we are considering code quality, which I consider important if you are actually going to be maintaining it yourself as oxide will have to do since they need customizations, then most of the proprietary Unix sources are just superior imo. That is, they have better organization, more consistency in standards, etc. The BSDs are slightly better in this regard as well, it really isn't a proprietary vs open source issue, it's more about the insane size of the Linux kernel project making strict standards enforcement difficult if not impossible the further you get from the very core system components.
Irregardless of them being ex-Sun (and I am not ex Sun), if I needed a custom OS for a product I was working on, Linux would be close to the last Unix based OS source tree I would try to do it with, only after all other options failed for whatever reason. And that's not even taking into account the licensing, which is a whole other can of worms.
However, if we are considering code quality, which I consider important if you are actually going to be maintaining it yourself as oxide will have to do since they need customizations, then most of the proprietary Unix sources are just superior imo. That is, they have better organization, more consistency in standards, etc. The BSDs are slightly better in this regard as well, it really isn't a proprietary vs open source issue, it's more about the insane size of the Linux kernel project making strict standards enforcement difficult if not impossible the further you get from the very core system components.
Irregardless of them being ex-Sun (and I am not ex Sun), if I needed a custom OS for a product I was working on, Linux would be close to the last Unix based OS source tree I would try to do it with, only after all other options failed for whatever reason. And that's not even taking into account the licensing, which is a whole other can of worms.