those are both really vague. what doesn't work and what is missing that anyone really cares about?
Hardware support: mostly cards from various vendors who won't release open specs. Like Adaptec, nVidia, et al. For Linux you get vendor blob drivers or quote open source unquote drivers written under NDA in which the actual functionality is obscured. Or various things on laptops don't work, or whatever. These limitations are not much problem for me, but they bother some people. If I build a server I spec it out with compatible hardware. No big deal.
For many applications none of the above matters, or matters a lot less than the benefits gained. I like OpenBSD and use it for servers and workstations. OpenBSD has pros and cons, like any OS. For my usage, the pros are a long list and the cons don't matter much. But if I were tasked with building a processing farm with tons of cores and memory to run a massively threaded crunching program then I'd pick something else. But I'd still keep the farm walled off behind OpenBSD.
* Secure and functional out of the box. The base install comes with many common services ready to go, and I don't have to worry if I turn them on.
* Simple, understandable. With a little learning, I understand how my systems will behave. I love it that the man pages are present, current and thorough.
Are there downsides? Yes, of course there are. Hardware support is often lagging, and some OS features are still missing that have been in other OSs for years. OpenBSD is a smallish project, and a few dozen part-time devs can only do so much.
So do I recommend that everyone use OpenBSD for everything? No. But I think it's worth learning and adding to your list of options. There are roles where OpenBSD is simply the best choice available, and roles where it's either poor or plain unworkable. To discount it for anything because it isn't best for everything is a limiting viewpoint.
Lastly, anecdotal... of the people I know who are really actually familiar with many OSs (Windows, Mac, various Linux, various BSD), all of them like and use OpenBSD to some extent. These are people who know their options and will choose what they need to get the job done well.