Bringing the alc interface corresponding to the Mikrotik card can sometimes wedge the whole machine - the SSH session to the box dropped, and testing on the local console revealed that the onboard em(4) interface stopped passing traffic, and ifconfig hung any shell until I rebooted the machine.
This card's interrupt status register does not clear on write. Reading it back after acknowledging INTR_RX_PKT0 returns exactly the same bits that were written. alc_int_task() re-reads the register to figure out if more work arrives, but it always finds the bit it just wrote, and re-queues itself forever.
Skip that check on the Mikrotik card, as there is no way to tell if a bit we just read is stale or fresh. Additionally, we now acknowledge only the bits we actually handle, rather than writing the whole register back blindly, and have a per-softc interrupt mask so this card can use a different mask (in this case, to add interrupts that report the link state, and in the future, handle multiple RX queues.)
The card does not yet pass traffic, but it is a step in the right direction, and prevents a system hang when the alc interface associated with the Mikrotik card is brought up.