Yeah, I think I only ever came across MCA one or twice? On a couple of PS/2 systems, which, IIRC, was the only place you could find it. EISA was totally backwards compatible with ISA, as it was just a deeper slot with a 2nd set of pins, it was more common, but not like super common.
I worked for a place that had the contract with City of Manassas, and all they bought were PS/2 systems so I had a bit of dealing with MCA. (IBM used to have a plant in Manassas, which is probably why they used PS/2 systems).