I'm using My Traffic Professional, which I bought during some sales.
Installing it in P3D requires some special steps which are described in the documentation. But if done right, that traffic will work quite well, and includes some military traffic as well.
If your CPU is old like mine (i7 960), that AI traffic will have some impact on the FPS at major airports.
Also, it will make sim start time a bit longer, because of all of the additional AI aircraft models in the hangar (not shown in the aircraft menu, of course, but the sim has to load them nevertheless). That means it takes longer to reach the initial menu of the sim when you launch it. This happens with any AI traffic, of course.
On the freeware alternatives, any freeware AI traffic will work just fine in P3D. The problems are the same as in FSX: AI flightplans in FS9 format will cancel any FSX-native traffic. Flightplans need to be converted to FSX using some freeware tool. Also, all freeware AI traffic packages are using FS9 models, which is heavier on FPS. Some payware AI Traffic packages are using FSX native models, and My Traffic Professional is one of them.
That payware traffic package is just a normal AI traffic package though. It's nothing smarter than any other AI traffic package, freeware or payware. It's not something that uses any special dll or external program to control the AI planes or anything. AI traffic will be as dumb as any classic AI package. But that's fine for me