I tried it out with marignon df, and I think it can be a fun sp strat but not worth it for mp. Even with high luck you do not get that many good units for the blood. The weaker ones are better than milia, but not by enough to spend your blood on them. I cannot think of a nation that would find them useful.
If you are playing for fun, the grotesque looks worthwhile to GOR, as well as the ettin. Maybe the chimera, draco lion, or cockatrice, since you are just playing for the heck of it
I was trying to think how to mod crossbreeding to be worthwhile. You could lower the blood slaves and/or research level, yet another way to generate fodder isn't that great. I'd rather see the units be more interesting. More protection would be a big help, most of them have less than 10. More abilities would also be a plus.