Yes the default designs aren't very good. They tend not to have many traits. Traits are a cheap way to make units noticeable better. For instance archers with accuracy and fast are much nastier. Also some units like defenders are just bad. Design a defender with a sword instead of blunt weapon and they are much better. The AI has access to many designs players can't access also by the way.

Thanks for the replies.

I didn't realise you could actually choose the traits and accessories for them - I thought that was just for champions.

I'll definitely give it a try now.

Would really like designed units to be dumped into separate subfolders though headed by the faction name. Currently, all units are put in the same place and it's hard to keep track to which unit is for which faction.

If you play larger maps, you'll definitely want a broader selection of mounted units than are offered as standard, particularly cavalry with ranged weapons. You can start training these fairly early - you'll need to research horses and enchantment (unlocks ice staff & incineration staff). I find that Champions tend to make better melee units since weapons and armor far better than anything available to your trained units in the early game can usually be found in goodie huts and lairs.