Trained unit level up bonus

They get the same bonuses everyone else gets:

+1 Accuracy

+2 HP

+1 Spell Resistance

+1 Spell Mastery 

Other than HP,
1 spell resist per level

1 spell mastery per level

1 accuracy per level

Also, in the senario, trained units receive 2 accuracy per level.

So, no bonus on Strength or trait? That is too bad

Only if the traits specifies it (Some traits say "+1 Dodge per level" or similar)

Sincerely
~ Kongdej

Once a trained unit got to level ten, being able to select an additional trait would be great.  It would really change how you use your trained units.  And I still think you should be able to ugrade their unit size as those technologies come available.

It's unlikely to happen. I hope that I am wrong, but looking back to the Betas the trend has been toward reducing the importance of leveling up as opposed to increasing it.

For example, the level up bonus for HPs got reduced, some of the armor level bonuses are gone as well, and the grave elementals and butchermen's numbers have been reduced. And the latest - the lightning pike, one of the few weapons that actually gave a bonus for veterans got nerfed.  The developers do not want troop levels to matter too much, so they are bumping off anything that benefits from levels. The champion lance, acrobat, lightning hammers?  I have little hope they'll remain unchanged for long.

I think it is a terrible idea, and the main reason summoners are so bad after the early game.

Thanks for the detailed answers