I have sent a few expeditions, about 50% get lost and those that return bring very little. Are they worth doing and if how?

Sharingan2, you can use Queens to increase your survival rate.
Expeditions are kind of a lottery. You're not supposed to gain stable profit from them, but you have a chance to find some unique artifacts in the unexplored space.

Thank you.

Does it matter how many units and what units I send on exploration mission? (except Queen)

Sharingan2, resource bounties are proportional to your fleet capacity, so sending cargo units should yield more resources in case of success.
Some artifacts (e.g. http://xcraft.net/infos.php?gid=97 ) require exploring specific sectors, if you want to obtain them.

Thank you.