I can't speak for chrony but considering the amount of security issues with ntpd I would much rather run openntpd.
That means it's more suitable for following a reference clock, not keeping a number of machines in sync with each other and the rest of the world. Unless they have implemented more of the protocol now. This has security implications of its own.
I've heard other people claiming that OpenNTPD is not accurate enough and what not, but my anecdata says it performs well enough. Do you happen to have any specific gripes with it ? Is it the timekeeping algorithm that is lacking ?
Sibling comment mentioned leap seconds, and that's unlikely to change. AFAIK, for 'proper' leap second support, the daemon must support it (to parse the announcement from the upstream NTP) as well as the OS. Gathering from I've read on OpenBSD's mailing lists, leap second support in the kernel is not a priority -- to say the least. Seeing the fallout on other OSes, I'd say it's a sound decision. On a similar note, Google introduced leap smearing to not deal with introducing leap seconds across all of its' servers[1]. Several other actors, such as Amazon[2] and Akamai followed suite.
[1] https://googleblog.blogspot.fr/2011/09/time-technology-and-l... [2] https://aws.amazon.com/blogs/aws/look-before-you-leap-the-co... [3] https://blogs.akamai.com/2016/11/planning-for-the-end-of-201...