Archer and slingers are ok in almost every aspect to me. I like to close up on the target unit whenever I can and most misiles will be deflected by armor and shields when present. I have even seen foot signifiers, the guy to the right of each unit acting as non-com/officer, take shots and keep standing, which I like to think of as 'injured but efective'. Most projectiles miss or are deflected for what I can apreciate.
I can even accept friendly fire while moving or in a melee situation. But what really needs tweaking imho is when a archer/slinger/skirmisher unit kills a friendly standing not two feet away!! It happens to me every time I'm not careful not to overlap unit formations. On one battle I lost 4 hastatii men that way. They were overlapin an archer formation and the 4 archers behind them shot them dead on the first volley and the enemy was at extreme range.
That's one of the many examples I've seen. Which is also a fine way to increase exp. chevrons on foot misile units. Just park a peasant unit directly between the enemy and your slingers and they'll be cut to ribbons as easily as if a gatlin gun had been behind them.
Unlike MTW I never position misile units behind others with the 'fire at will' button on. It's too dangerous.