The most common OS are very heavily tested because of the user amount. These "secure" operating systems have niche amount of users which further reduces the amount of testing. And this is the only helping factor you - it is more beneficial to target operating systems which have a larger adaption. You need to be on high priority that they start developing exploits only for you who is using some random OS.
By comparing the number of exploits? Qubes relies on Xen, which is used by very big targets, so should be under constant attacks. Qubes uses hardware (VT-d) virtualization, which AFAIK was last time broken by the Qubes founder in 2003: https://en.wikipedia.org/wiki/Blue_Pill_(software).
This is often giving quite misleading conclusions based on what I just said - iOS for example is much more popular and heavily tested - of course the amount of exploits is much larger, because it is also much more interesting target as many are using it.
How many people are using phones/laptops which are based on Xen? Xen is commonly used on server side - not by those guys who are holding the interesting stuff on their personal devices.
AFAIK server side is often even more interesting for hackers as it's connected to big money.