Yeah, I've been happy with my SoloKey, but OnlyKey's integration with a software password manager + OpenPGP + SSH keys is really enticing. I'm on the same boat as a lot of others here, however, that the lack of open hardware is a deal breaker.
Open hardware has the benefit of being able to build it yourself, which is the only completely secure option. The downside is, indeed, the ability to easily create malicious clones, and the fact that you simply won't be able to build it yourself for any remotely modern hardware. So yeah, there's really no security benefit to it in terms of hardware.
Proprietary hardware has the upside of needing reverse-engineering to create a malicious clone / part, and the transparent design helps you make sure that they can't do a sloppy job at it.
It's a shame that tradeoffs have to be made once technology reaches a certain level of complexity, but alas.