Actually, the fact that the weapons don't one-hit kill you is easy to explain via the shields. That melee goes through shields is logical/illogical dependant on how the shields work.

The reason melee weapons in real life are far less effective than ranged weapons is not that they -don't- do more damage (in most cases they actually do), its simply that ranged weapons can quickly take a target down before they get anywhere near to melee'ing. That and its safer to stay behind cover.

When you bring shields into play that can hold up to ranged fire for enough time, meleeing your target can actually become a valid tactic, especially if the shields don't work against melee. Unarmed combat against combat armor does seem a little wierd though.

There are two illogical ground combat issues that I can see. One, how does expose work against a target with a shield up? Two, why does it take multiple hits with 25th century weaponry to take down an unshielded target?

However, since game mechanics do not need to conform to logic those issues are merely aethetic/immersion breaking. On the other hand I wouldn't mind seeing shields buffed and health reduced proportionately so when your shields do go down its like "Oh, crap".