I wouldn't hesitate to agree to a flush back if it was the only way to get my hands on some desireable genetics and it helped get the price in my price range. For smaller breeders wanting to get their hands on females that are otherwise out of reach, I think this is reasonable and can be beneficial to both parties.
Personally, I would want to enter into that kind of contract with someone I was familiar with and trusted. Furthermore, if I was able to convince someone to entrust me with their best genetics, I would be appreciative enough that a flush back would be a sort of thank you gift in return.
Technologies are improving to the point that flushes don't necessarily have to disrupt your breeding schedule. Donor can be bred on schedule and an aspiration pick up procedure is available, non-surgical, that can actually be performed on a pregnant female with no risk to the developing pregnancy. TransOva does this and the IVF embryos are now of the same thaw method as regular type embryos. You can even reverse sort the semen and select the sex of your choice in the eggs, or use two different sires if you have enough eggs to work with. $1300 for aspiration with reverse sort...yes, it's pricey, and not saying this is available to everyone due to where you are located, but it is an option to think about.
I just purchased a heifer with flush back terms. I negotiated with the seller and depending on the harvest, we may split the flush or split two successive flushes utilizing two different sires. I wouldn't rule out a good animal simply because the seller is asking for genetics back down the road. If the seller is a jerk you don't want to deal with, why buy from them to begin with?