[F. Gottwald]

soloing isn´t really much harder, it just works differently from playing with a party. different parts become difficult... a few examples:

greater wolfweres are horrible when you´re on your own... very very very hard to kill them.
a single hold/stun might end the game, a single charm/domination/maze WILL end the game. maze is especially annoying, shield of harmony and similar items help very well against the others.
you might lack thief skills... traps can become a problem, locks usually aren´t (you automatically force all game-critical locks, and which truly great item is locked away anyway? i can´t think of any)
a lack of massed mage attacks makes it more difficult to disrupt enemy spellcasters

certain battles become easier since you have to look only after yourself... so in spell battles you can turn yourself into a mustard jelly and laugh at those silly mages.
you can´t accidentally hurt your party members with area effects
it´s easier to run away or sneak past something you don´t quite like how the battle is going
your game will run smoother if you have a sucky graphics card like i do
you will amass experience much more quickly... in some instances one high-level char might be stronger than 6 medium level chars (epsecially if the solo char is a wizard or sorcerer)
you only have 1 char to equip - money is less of a problem


about the resting bit... there are tons of regeneration items available in the game, after you obtain the first you never need rest for hp, and spells last fairly long if you don´t use them unneccessarily (my sorcerer only really rested when he was fatigued anyway... a wizard might wish to rest a little more often though)
on the whole, it´s far easier to solo than most people who haven´t tried would think... especially if you consider that you don´t have to butcher everything in your path. and if you have to fight, summoned creatures are yoru best friends (for non-casters, there are several useful items for this)