On legendary....
Physical attacks.....at high levels (50+) the game will be sending 90% top level foes....eg Deathlords and more than the usual numbers you get at lower levels. You can easily get over run and mobbed by tough to kill foes.....so you might just be needing that legendary smithed armour to be able to survive after all eh!
Magical attacks.....legendary will TRIPPLE the normal (Adept) kill power of a mage's thunderbolt and Dragonbreath etc, on you! Your super armour provides zero magical protection.....so you might just need those enchantments and potions eh! It will feel like this.....imagine playing on adept as normal.....but with every necromage and dragon killing you with three times stronger power.
But it is a rewarding challenge to STAY ALIVE on legendary....if you like rewarding challenges. Mods will add a lot to the experience....but I'm just talking vanilla here since that's the comparisson base line.
However, if you ask me what difficulty you SHOULD choose....it depends upon your build and preference. What you are meant to do is make a char build you like....then adjust the difficulty slider to find the sweet spot where YOU find it most enjoyable to play.
Some people like it tough, some people prefer to cruise through laying waste to all before them, some people reload a killed char repeatedly without a qualm, others hate reloading viewing it as their failing the poor char. Some go to the extreme of never reloading a char....they fight for all their worth to keep the char alive, but if they are killed thats the end of the char permanently (Dead is Dead).
The choice is yours