Question: "Will a wooden knife handle be less durable than others? Is it safe to put the knife to hard use?"
While all wood is beautiful in it's own way, some wood is simply not suited for knife handles. Natural materials that make the cut have to be resilient, durable, stable, and attractive. Wood handles such as Arizona Desert Ironwood, Snakewood, and Lignum Vitae are by nature incredibly hard materials -- if you've ever had to cut Ironwood you know where the name comes from! These species of woods make fantastic knife handles. They not only accept abuse, but ask for more.
More important that durability for most knife users, is stability. The materials previous mentioned are are dense enough that they are naturally stable and not as susceptible to warping or shrinking as other natural materials. Softer woods that are not naturally stable require an industrial process before they can be used for handle materials. This process involves impregnating the material with a resin substance, think Cyanoacrylate Ester on steroids. This substance fills the pores of the material and 'locks' it in place. This stabilization process adds to the durability of the wood, prevents the wood from deteriorating, and also adds to the brilliance of the wood when it is polished; particularly after being sanded to a high grit.
Stabilized wood can still warp if taken to a drastically different environment than what it has been acclimate to, it's the nature of the beast. If this happens, it's not a worry! Contact the knife maker or knife company to have the handle re-finished, any reputable knife company should warranty this for free.