Well, I have 3 of the 4 you listed and can give some general feedback. The comparison would be more with the FX9 vs the PC Carbine (both taking 9mm generally), but I can say this...
WK180C - reports of various issues in earlier generations including sheared charging handle and snapped piston rod. I have the generation 1.5, I guess, where it is not the original one with the magnetic handle (mine is the darn threaded one which comes loose every time I shoot it). I haven't had it shear off (yet). As for the piston rod, thank goodness a fellow member recommended I check the gas block screws. Sure enough, mine were as loose as my ex's moral compass, and so tightening the screws and applying some locktite may or may not have mitigated a problem later. As for accuracy, it is decent out of the box. Weight and feel is good. Takedown is relatively easy, except that hairpin retaining pin thing on the bolt for mine is hard to put back in. The whole thing does have a rather odd rattle from specific locations I have been unable to pinpoint. There is no bolt release/lock on it (though TNA has a kit for it). I have a Lantac Dragon MB on mine and it shoots great. From an enjoyment perspective, I like it. Can shoot 5.56 and .223 without issues. Overall cost of ownership will be higher per round due to ammo cost.
Ruger PC Carbine - mine is the one with the aluminum handguard and extendible / adjustable stock. Overall, I like it. Can take it apart in two pieces for transport. Fairly reliable overall. Reminds me of a souped up 10/22. However, there are some detractions (for me anyway). Due to the aluminum handguard, the weight distribution is not great with a noticeable heavy front. Even a foregrip doesn't alleviate the noticeable weight difference. While it can take apart in two, taking it apart to clean is a pain. Yes, it's only two screws, but you need tools for that. Then tool to take the charging handle off. After that, the design of the shock buffer/clip makes the shock buffer bent every time - I swear it was gonna snap - the circular piece is plastic as is the clip. MCARBO kit here is a must. Some people don't like the look of it, and I agree to an extent - it does look rather bulky/cluttery. Enjoyment is decent, but not as good as my...
Freedom Ordnance FX9 - I have taken this to the range more regularly than any other gun I own. Ergonomics are good. Weight is great. Takedown is so easy - no tools and it is a breeze to take the bolt assembly apart. Accuracy is good and I shoot mostly 147gr in it though it's fine with lighter bullets. It comes dry, so oil it up good. Only areas of improvement - last round bold hold open is hit and miss with the Glock OEM mags. The bolt catch goes off by itself. Charging handle - even the "enhanced" one - is too small - a guy on here sells a larger one. But aside from that it is great to shoot, and looks superb to boot. FO says not to use aluminum ammo, so I haven't.
