Yes, balsa can be very hard. it ranges from almost as soft as white styrofoam to almost as hard as spruce. The most common stuff in kits is a medium grade, a bit on the light side.

It can be difficult to hand slot the hard balsa for hinges. The Great Planes electric hinge slotter (or a Dremel cut-off wheel and a VERY steady hand) can help a lot. I've also been known to use a small drill bit... punch a bunch of holes on the hinge slot line, and then get out the DuBro slotting kit. The holes make it much easier to punch the forked tool in, and then picking out the crushed balsa is fairly easy. Modify these techniqes based on the hinges being used... its easy to end up with a WIDE slot which is no good for CA hinges.