So I have now successfully played through as both Good and Evil Ironman Mode. Evil was Drow Rogue and Good oddly enough was a Tiefling Warlock. The main difference that I came across other than story elements and playstyle was that by going evil you effectively knock out any need/reason to go to the Underdark.
What about the Nightsong? Search for powerful items, allies and wealth? The Underdark has a TON of power and wealth items.
Arcane Tower
Kuo-Toa
Spectator Beholder
Ormeluum the Mindflayer
2 vendors In Myconid village
The Only Purple item in game right now - also in Myconid village
Bullet
Hook Horror encounter
Boots of Speed!
Again I point out what I had said. There is nothing Story wise that pushes you in that direction. There is the whole "
For Honor and Glory" bit but otherwise you are sent over the land to get to the same Towers led by a Drider. ( My guess based on the dialogue with Minthara ). You wouldnt even really know that the Underdark exists depending on some of your dialogue choices with a few Goblins as they mention prisoners being taken.
The other problem of XP capping before you ever reach the Undedark is a separate issue as the main topic was the difference in paths when you go Good vs Evil.