-To be brutally honest (Hopefully non-offensive and on topic)-
Guns are far easier for tripwire to implement into the game. They require a smaller amount of code and not as many anims. (But don't underestimate how much work goes into one! Marco's work on the chaingun shows that) Tripwire would have an easier time adding a modified weapon to the game which just appears in trader time and does decent damage the community may dissaprove, but all they have to do is not buy the weapon, unlike specimens which must be faced in every game. The hard part is making sure the weapon actually adds something new to the game.
Specimens are difficult to make, and when you're done, there is no guarantee tripwire would accept it. Models could be judged as too scifi or just not fitting with TRIPWIRE'S vision of the game. (Not just the maker's opinion counts when it comes to having it accepted) A successful specimen though would be impressive for the community to take on (aka the brute). However, the current engine has some major restrictions in terms of what you can have for ai and while the creators put a lot of work in, the community may not realize this and see the specimen as incredibly primitive. The reason I say this is all specimen creators I have met in the past have ended up being scared away by these various reasons.
Gametypes are the hardest to make of the three depending on what the goal is. If it's a slight modification like the roll the die, then it can be a lot of fun. There are also a lot of mods made by a good friend of mind who sadly, no longer mods KF except privately named BlackCheetah- including the specimen VS and the original AI allies mod. Coders are very hard to find in this community but turn up a lot more on the unreal forums. The existing mods are not always posted on the forums, but if you join those small servers no one seems to play on, you may be pleasently suprised.
Quoting Zynthetic from one of the times we talked:
"In the scope of balance it's easier to add things that kill than add things to kill."