Agreed. I haven't done this, but I have talked to a few guys who have, and seen their results. Broadly, the smaller the piece, the fiddlier it is to get on, and consequently the less slick the result. The smallest pieces were fine for a covered or semi-covered hairline, but they would not have been undetectable enough for a fully-exposed slicked-back style. If you want to do that I think you would need to shave off your forelock and get a lace front partial maybe two or three inches deep and including your whole hairline. That would work well.
It might be different if you are Ben Affleck and you have someone to help you put the piece on, but most guys don't have that.