I've owned a few Impressive bred horses. But personally I wouldn't breed an Impressive bred horse that was N/H or H/H.
I do still have an Impressive bred mare with the linage. I researched her pedigree and found it's 6 generations back so she's got what- a few drops of Impressive's blood- that doesn't bother me.

As long as the horse doesn't carry the problem, N/N horses don't scare me. I've found every Impressive bred horse I've ever been around or owned were all very smart, VERY quick learning, and willing horses. I'm not run off as long as it's a neg. result in the HYPP test.