Not hard at all. However, there are hackamores and there are hackamores. These things can be rather innocent and they can be also torture devices. A general rule is - the wider and the softer the noseband is, and the shorter the shanks are, the milder the hackamore. Avoid hackamores with thin, sharp nosebands, with chain chin straps and with very long shanks. Hackamores apply pressure on the poll as well, not just the nose - some horses prefer that, some don't. They cannot be ridden in strong contact and they are NOT for beginner hands, also, they have to be fitted correctly, so that the sensitive bones in the nose and facial nerves are not damaged, if the hackamore is set too low.
A good choice to begin with would be an English hackamore or a little S hackamore (my choice which I use daily) - and you can also look into LG hackamores, Flower hackamores, Orbitless, and others, which can be used as a hackamore and easily transformed into a sidepull.
And, as always, a piece of tack is only as harsh as the hands that are holding the reins. But hackamores are more likely to become harsh, even the mild ones, if they are used without a feel.
I have come a long way, to surrender my shadow to the shadow of my horse.