I've run into it in a second or more remodel of an office remodel. I haven't answered since I didn't open it myself. My experience is that it is strong block just like concrete except it chips easier with a hammer.
The holes cut for ductwork didn't have any reinforcing and the door opening just had a 2by lumber frame with a standard 2by header.
If you are just cutting holes for gable end vents, then use a circular saw with a concrete blade to cut what you can and break the rest with a hammer. Use 2by lumber at the thickness of the block to box the interior hole. If you are cutting out the brick too, then you may want some steel flat bar to hold that up.