Unfortunately, going only based on what is presented in the show, they’re rather dull. Most of them are evil for no reason other than a lust for power and lack any backstory to explain that drive.
This is not the first time I’ve heard this. My question is: How complex should the villains in your opinion be written in a family-friendly cartoon? If you consider that for most villains only one or two episodes are available for character, plot and defeat, I find it understandable that usually only quite trivial motives come out.
And even if we like to forget it sometimes, in MLP there is still a certain cartoon logic. The villains are so stupid or one-sided characterized, because it also makes the protagonists look better and makes it easier to defeat them. That’s not the “guilt” of the villains, is it? ;-)
And their influence is barely felt on the world outside of the episodes they appear in.
Well, I think we all agree that MLP has barely managed to handle its own lore in all these years. Villains and also later important characters like Shining Armor appear almost always out of nowhere, at least we have never heard of them before and even things that are mentioned simply remain without any further explanation such as Saddle-Arabia or the origin of the Crystal Empire. I would put the influence or even a lasting impression of the villains on the world of Equestria at the very bottom of a list of problems of the show. ;-)