Of my Omega and Zenith boar brushes, a couple of which I've been using between 5 and 10 years, I've managed to get only one Zenith to my desired state of split-ended fluffiness. I've inherited others, which have been used for decades, without problems. All in all, Hogsnort Rupert and his Famous Porkestra have never played me false.
I've found that any natural fibre brush will, eventually, wear down and become shorter, after many years of heavy use, but I've not had any immediate "failures" of mine. Then again, I follow the principle laid out by
@hotmetal and
@pjgh and break them in (eventually), by normal use. The only pre-use treatment that I've seen advised, is a light shampooing or two, to eliminate any odours. However, that's usually not necessary with any of the modern boars, which do not come pre-stunk, and the only thing in relation to this that I think might wreck a brush, is boiling it.