Which version of OPNsense did you try? 16.1.x or 15.7.x?The first has a higher kernel version. Might be worth giving it a try with a 15.7.x build.
Which pfSense version did you use? pfSense 2.2 is on FreeBSD 10.1, and we've seen a lot of VM regressions with FreeBSD 10.2 that we switched to with OPNsense 16.1.