Am I just really bad at using the editor, or does it have a lot of annoying problems? I keep going to use a set of tiles (for example; mitte Building 1) and instead of fitting together easily, I end up with holes and misalignments. Heck half the colours don't even match.
I can't make ornaments go on a wall for the life of me either, they end up clipping though unless brought several squares out so they're floating in the air. I've done the HBS tutorials and frankly, they weren't that much help.

Any suggestions or tips would be very much appreciated.

I think the HBS forum would get you more help.

Doesn't seem like they have a forum.

This is what I vaguely recalled: http://www.shadowrun.com/forums/categories/game-2

Dunno if the modders that used to hang out there still do. If they do, then I'm sure they don't strictly talk only about Returns.

Or now that I think about it, you could probably get someone over at Nexus to tell you how modders communicate with each other, now days.

Yeah, the modding community is pretty quiet these days, but the forums at shadowrun.com are probably your best bet.

To answer your specific queries: it's not just you. Unfortunately, a lot of building tiles just don't fit together well, even if all taken from the same set. As with many things in the editor, it's usually best to open the official HBS campaign and see how they did things. There are some assets in the editor that they don't use themselves, or only use in very limited situations. As long as you follow their example, things will probably look OK. Otherwise, be prepared to spend more time experimenting and swapping out things that don't work.

Positioning objects against walls also tends to take some trial and error. Experiment with moving items parallel to the wall along the X or Z axis; often, items will clip in one position but display fine if placed 1 unit further down the wall. Moving items up in the Y axis also often helps, but try to make sure the X/Z location is still realistic, otherwise it may interfere with movement and lighting.

My biggest suggestions are:
(1) Study how HBS builds their maps and try to follow those patterns.
(2) Use dark lighting. Gaps and seams are much more noticeable in bright light than dark.
(3) Add cover. If something looks ugly, place something else in front of it so it's less noticeable.
(4) Don't sweat it too much. After you start looking at this stuff closely, you'll start to see all sorts of imperfections in the official campaign maps; but those are things that 90+% of players will never notice. Especially if you're using dark lighting and have a good atmosphere, most players won't even realize that some items are clipping or that there are seams on the wall.