I'm A Banana
Potassium-Rich
This left me scratching my head. Why would an illusionist suck in combat?
Oh sure, they might have low HP, lousy armour, and not be terribly good at sticking the pointy end of something into the other fellow, but the ability to alter the enemy's perception of the situation is a devastatingly effective combat ability.
Making the enemy see what you want them to see, while keeping them blind to the things you want hidden is a fundamental aspect of the art of war.
If I can make the enemy flee from an overwhelming force that doesn't exist, around obstacles that aren't there, into clear terrain that is actually the edge of a cliff … I'd say I'm pretty darn sensational at combat.
Combat is more than HP, to-hit, AC, damage dice and number of attacks. You can suck at all of those things and still be very effective in combat.
Hey, they might not! I was going with the idea that in a D&D style universe, there's typically a lot of things that won't respond much to your illusions (thinking in terms of wild animals, monsters, divine beings with illusion-piercing vision, etc.). But I wasn't trying to be definitive -- maybe your illusionist would be a decent combatant AND a decent face, and suck when he's forced to rough it in the wilds! Or maybe he'd be fairly well-rounded on all pillars. The point being that it should be OK to suck at something if that's what you want to do with your character (and the counterpoint that it's OK not to suck if you don't want to, too!).