I think it is much easier. If you are in climb your momentum vector shows up and best glide assume vectors leveled. When immediately going to best glide velocity the momentum part up in vector addition will be eaten up by gravity and you are too slow horizontally - boom. So, first level and adjust attitude before going for best glide, but who takes this time in such a situation? Everybody should be able to flare and land from powerless best glide approach, or should train this specifically.