I don't think they are. My main is an assassin, and I soloed her almost exclusively to 90. Yes, it was hard sometimes and yes, there were a few mobs that I just couldn't take. But it's doable.
Obviously you want to make sure you can sneak up on the mob and open with your most damaging stealth attack. Try to make sure those abilities are expert or master if you can get them. For the mobs that see through stealth, make use of your bow abilities (also need to upgrade them as much as possible). And during the fight, stun the mobs, then hop behind them, use that one ability (can't think of the name right now) that puts you back into stealth again so you can use another hard hitting backstab.