It makes your claim fabrication less likely to be discovered, thus causing less AE for fabricating one on average. That can be quite useful, but if you're only fabricating a couple claims on a rival, the difference will be very small. It also works on rebelsupport and the various actions you get from Espionage ideas, but those are a lot less commonly used.
Diplomatic reputation and 'better relations over time' are very good general purpose diplomatic advisors. The rest can be much better or much worse depending on your situation. I'd usually go for 'better relations over time' over spy offense though since it also reduces the effect of AE, but not just for claim fabrication.
100% better relations over time from Religious Ideas is possibly the 2nd best overall idea (next to Forced March). Covert ideas are greatly underpowered.
For example, Russia succeeded in
Sabotaging Reputation on me. Suddenly everybody was a lot less friendly (-50 malus). But, for it to work they have to keep a diplomat permanently my capital, and I have the Spy Agency and Cathedral built in my capital which gives me 35% increase to spy defense, so after a few months, their diplomat gets discovered and sent home, and the malus disappears.
So, it's useful, but limited because it ties up your diplomat for far too long, and because for a major power, it's bound to be discovered so it's not permanent.
