mostly depends on what kind of filament you use to make them, PLA is cheap, and looks pretty good, but you can break it pretty easy, and it doesn't stand up to sun or heat well. PETG is stronger, and holds up better than PLA, while ABS is the strongest and most durable (they make motorcycle helmets out of it) but it takes higher temperatures to print well, stinks while it's printing, and the fumes are just mildly poisonous....
and on the kind of interior support structures you use. you can print something as a solid chunk, but it takes a long time, and uses a lot of filament. most 3d printed items are mostly hollow with support structures here and there to supply solidity. if you look closer at the nut and bolt you can see them inside, as i printed that from "clear" PLA