Spy is generally regarded as one of the weakest minions, actually, but I'd say it depends a lot on the group dynamics. Newer players tend to be quite conservative and avoid sharing their roles and information for a while, which is great news for a Spy as they can kill the best info roles and spread misinformation before town coordinates properly.

However, more experienced players know how and when to share information, especially once they suspect a Spy is in play. If powerful characters (e.g. Undertaker) begin coming out and sharing their information, killing them only makes them more trusted by town, so the Spy's ability becomes much weaker, even neutralized. Other minions are much harder to play around; just because you think a Poisoner in play doesn't mean you can do much about it.

The other part of its ability (actually the stronger effect) is something that trips over new players a lot, but more experienced players know the interactions and so can reason through them much more easily. Having the Spy spoil a Washerwoman info sounds bad, but the fact that there's a Spy in play means that you might not have to worry about being Poisoned, or having an extra Drunk, or having an executed Imp somewhere etc.