I've kinda wondered about that, too. They do in a way have kind of a flimsy appearance to us, but clearly they are at least space-worthy. It's obvious to say that, but its actual definition is vague.
The range question is hard to address. In the original series we saw Kirk and Commodore Mendez attempting to catch the Enterprise
in one, but there were references to the fuel supply running down, though we have no idea how far they'd flown at that point, though it couldn't have been too long.
On the other hand, Lokai of Cheron had stolen a shuttle from Starbase Two (or was it Four?), and it was stated that the shuttle was stolen two weeks ago
when the Enterprise
had run across it.
Presumably they could do more by the time The Next Generation
had come, but I've no idea how much more. They were sometimes seen to be used for rather long journeys, so they must have been reasonably durable. One can assume they had replicators, so people would have access to food and water. As far as sleep...I would have to conclude that they'd have to snooze in the pilot seats.
Defense-wise, I'm not so sure. I seem to remember the Romulans had no trouble immobilizing Geordi's shuttle in The Mind's Eye