One of the first things any 3d modeler should learn, in my opinion, isn't how to use all the bells and whistles of their modeling app, or the mysteries of uv mapping etc...
It's far more basic than that, and comes in two parts...
1 Form follows Function.
Sure, architects design mad looking buildings, but... What shape are most houses, and why... Why are doors all around the world, slightly higher and slightly wider than a person... Why are elevator buttons always at the same height off the floor, etc.
2. Model what you know...
Any one can make to wheels, and a styled body shell to connect them and say "look a 3d motorbike", but if you know nothing about bikes, then odds are the first biker who sees it will say 'unusable deathtrap'. A friend of mine, a professional architect, who had made a highly popular series of over 100 futuristic architectural city blocks, tried his hand at making a gun prop,m despite knowing nothing about guns at all. Result was a gun where the ejector port was shorter and narrower than the cartidges... No way to eject the spent brass after firing, hace to pop the clip, and poke the spent shell out of the breech with a stick and shake the gun so it fell out of the magazine well...
So, think about the problems with building large structures in zero g and hard vacume, when every planetary rotation takes your worksite into the shadow, where the temperature drops to -273 degrees centigrade, then back into the harsh glare of sunlight, with no atmosphere to shield and tone down the radiation, where the glare can fry your retinas.
Your hull will be prefabricated panels, built in pressurised workshops, attached to prefabricated structural members also built in pressurised workshops, simple girder frames, with flat panels joined edge to edge, so, your uv's are going to be tiled, repetition of the texture is almost mandatory, either boxes because its a simple shape to build, or spheres because its the smallest and thus cheapest surface to enclose a given volume, or a compromise, the cylinder, because people dont like rooms where the floor is a mad shape and awkward to fit carpets...
Your sample models tell a lot, sketchup works well on face extrusion so thats what you use, but thats not how buildings are made, so your models dont look like buildings, simple as that.
Thing I'm most famous for modeling, if famous is the right word, is weapons props, especially swords. Why? Because before I started making swords, I went and watch a swordsmith...
Once had somebody who downloaded one of my free meshes post a reply "who models a full tang on a 3d blade prop'. I do, because thats how they are made, forgetting that there should be a tang means you forget how the proportions of the weapon should match up, it's like those moronic 'buster' swords you see in japanese manga based stuff, blade 6 feet long, 8 inches wide, attached to a grip thats thinner than a broom handle, snap right off when you lift it, assuming you can life a sword that must weigh at least 100lbs.
Making a gun mesh, first thing you make is a round of the ammo, as that will determine the whole size and shape and layout of the weapon, cartridge too long to pass through the grip, no through grip magazine, either a forward shift, for a mauser m96 look, or a backward shift to a bullpup design, like a calico 950. The length of the brass determines the length of the recoil stroke, size of the bolt, and the reciever, and the ejector port, how much propellant in the brass determines minimum barrel length to allow the propellant to burn before the slug reaches the muzzle, and so on.
With sci fi stuff, you have to think about who will use it, and what they will use it for. Take a look at the Destroyer mesh pictures I posted here in the gallery... Now you might not like the design but..., the reason it doesnt have a load of greebles on the hull is that such things would weaken the structure of the ships armour. I didn't model an interior, but if you ask me, I can tell you exactly how many decks it has, and what they are used for, and why C-Deck has no windows...
Form follows function...
Yeah, now and then an architect will find somebody willing to pay for a mad design with wierd sticky out bits in mad places, but... 99.9% of the worlds skyscrapers are rectilinear glass, steel and concrete boxes, because they are easier to design, cheaper to build, and more convenient to live and work in. Your carpet fitter doesn't need a degree in differeltial calcllus because your room's floor plans are based on hyperbolic curves...