Yes theoretically both Proleric and you are right. CPP can break things, even with just installing, if your custom content script relies on some unintentional vanilla script behavior/bug your script might not work correctly. And if you recompile all scripts after installing CPP its theoretically possible your scripts will behave differently.
Thats however only theory and while its possible to happen, chance that it happens is very very small. The former issue is to be expected - if you rely specifically on some bug (like it my example SignalEvent - Shield of Faith) then if CPP fixed that bug obviously your script stop working. I don't think anyone has a script like this. I don't even think anyone actually knew Shield of faith signals Camouflage spell ID and was relying on that. And if by any chance someone did, then is it such a big deal really?
As for recompiling scripts with CPP. As was said - CPP is designed to provide compatibility with custom content. If compiling your script with CPP includes will throw compile error or even change its functionality (which afaik nobody experienced yet) then its bug in CPP design and it will get fixed. If CPP was abadonware and there was no support, if you were ever hit some issue, I would understand the hesitation. But thats not the case I am around and I offer full support, especially for PWs.
But yes there will be changes, especially when regarding to AI as I informed already. If you don't want anything changed on your server/module, stay out of it. If you are willing to do some work for all those fixes/features CPP offers, CPP can be great resource. Due to the way it works its easy to disable anything. You don't like your AI being changed? Ok restore vanilla nw_c2_default* scripts. You don't like the new spell immunity/spell mantle mechanics? (spell mantle is now depleted only when the spell is not caught by spell immunity) use module switch to restore this functionality to the vanilla way. Etc.
@meaglyn....Thanks for the info, I noted that you can back out easily, if on Linux. I wonder how easy for Windows? This could be the make point for me. This way I can test and see the effects of the patch on my work. Pulling CEP is a nightmare, if CPP could be pulled easily, that would be excellent should I find I preferred not to use it. Oh ya, the list of info for the CPP is not light reading, lol, there's a lot to read in that puppy.
CPP can be removed by installing 1.69 critical rebuild or by replacing modifief xp2patch.key file in NWN folder with 1.69 version- If you install CPP with installer each overwrite file will be backuped (xp2patch.bak), if you install it using manual instalation you need to backup it yourself however the original xp2patch.key can be downloaded separately in CPP vault project page.
CPP overwrites 3 files only. xp2patch.key, dialog.lk and one of the texture packs in texturepack folder.