They say that you marry the individual and not the family. So if you don't get along with the family, or if they don't get along with your family, then it isn't their place to say anything.
Except you do marry the family.
Who I am is in large part a result of who my family is. I reflect them. I reflect how we grew up. I reflect the experiences we shared. I am my family and they are me. And the same is true of everyone. So when you marry a person, you really are marrying the family.
I dated one guy whose family I never really got along with. They never really liked me and thought I was the one who was the bad influence in the relationship. The last time I spoke with them (at his wedding) his sister wanted to give me a book so I could figure out what was wrong with me that I wasn't married. Talk about awkward.
It might be a small thing. I know people who are happily married and don't get along with their in-laws. But I also know people who don't get along with their own families, so there's that. The evil in-laws are a stereotype that I'd like to see go away. I know both of my parents got along with their parents-in-laws. My grandpas used to ask after the other one. And more than once I remember my dad's siblings asking about my mom's dad. It is totally possible to get along with in-laws.
Just like any relationship it takes work though. You've dated the individual rather than the family. Your experiences with them have been limited (which is why it is, except in extreme situations, vital to keep relationship issues within the relationship, that way when you get over it nobody else is still holding a grudge). And it takes time to build a relationship.
I met HP's family very early on, and he met mine very early too. His family still talks about that first meeting. I apparently made quite an impression. But neither of us lives near our family, so those interactions were rare. It has taken a lot more than those first meetings to develop the relationship to the point where right now I feel connected to them and can confide in them. It's taken time and communication. When he's talking to them on Sunday and I tell him to tell them I love them, I mean it. And the same goes the other way too. I've had conversations with his family that he didn't initiate, and he's had conversations with my family that I didn't initiate. And more than once this year I've told him how grateful I am that his family likes me. It makes so many things so much easier.
Now, not everyone can have that. Because while we are our family, we are also our own individuals. So it's possible to get along great with one family member and not the rest of the family. And some relationships can be stronger than others. But you do marry the family. And they are important. And in-laws are not evil.