not exaggerating at all.
As I said with the caveat of 'unless complex'
Most fsx aircraft developed with regular fsx systems will install manually, ie, installing to a created folder and moving the files in.
A lot of complex aircraft may require you to move some DLL's in, and some require entries in the p3d dll.xml file.
A program like notepad++ should open it no problem, simply install said complex aircraft to fsx, check in fsx's appdata/roaming/ms/fsx folder for its file, copy the entry in that dll.xml file into your p3d one, hey presto, in many cases said DLL's work.
I have all my A2A accusim aircraft, accufeel, and a couple of other systems heavy aircraft in p3d3 so far... no issues when installed correctly and external DLL's reading in the sim, that's probably the reason you're having issues.
The core sim itself is the same realistically... all you need to do is make sure things go where they need to, and relevant entries are active in config and xml files needing them.
simples... ish.