I've had constant problems with SEIV cards in 840av and IIci machines locking up at random during disk access. I've had 2 cards, I passed the first one (honestly) to someone else because I thought it was defective. I got hold of a second one and it does the same. The upshot of that is I use a Jackhammer in my 840av instead, which works flawlessly but isn't as fast.

My understanding is that the SEIV is designed at hardware level to take full advantage of the NuBus and Macintosh system, in whatever way was possible. Some of the techniques they used were not exactly 'by the book' and so sometimes they just don't play nice. I may try mine in the 7100cx and see if it works sometime.