That's what it pretty much always is. Outside of scutage (which you don't even need a hook for or to make concessions), modifying vassal contracts is a total waste. Yeah, here, let this guy have -15 opinion so he can be a pain in the ass during succession meanwhile I get a whopping what, maybe 0.5 extra gold per month? Utterly useless.If all you'd get is 0.1 extra gold per month, then I'd agree it's a bad idea to waste a hook on that. But that doesn't make the mechanic itself bad. The mechanic works fine, in my opinion, it's just in this case you're considering using the hook on something unimportant.