* mr is unreliable(?) Can you tell how you use mr?
We use it on RB450G to "virtualize" user CPE on some environments. Metarouter sees two interfaces: one is a dynamic interface bridged to the upstream interface on physical device (ether1, in this case), the other is a static interface (which may be ether2, 3, 4 or 5) which is the customer facing interface. There's some firewalling configured on the metarouter (nat masquerade rule, some mangle used for policing / shaping) and some queue tree rules. Nothing fancy. We discovered that layer7 matcher in the metarouter firewall can crash the whole device under some traffic patterns, since we dropped them from configuration the stability has improved a bit...but it's not here yet. It's quite frustrating because we didn't find a way to reproduce the issue, sometimes "just happens"; we're evaluating solutions from other vendors which is unfortunate to us, because we have 4k+ mikrotik CPEs installed and the metarouter is seen by our system just as another CPE (so we can use the same management / provisioning tools we used to date).
* mr not supported on CCRs and RB3011UIas
No, it's not.
* OpenWRT - no mr support in upstream project and builds provided by MK are obsolete
* OpenWRT project is stagnant, but there's a young fork called LEDE project
I can't speak about openwrt support since we never used it.