What's the beef with Grey Knights?
So, before anyone gets their knickers in a twist, yes, I've looked for similar posts, and no, I haven't found any that really answer the question I want to ask. If I've missed them, please feel free to point me at them (a link would be much appreciated).
I'm looking for an army to do, mostly as a painting project, but I like to be able to play with anything I buy. I play in a very small group, so don't want to field things that are going to spoil the fun for my group.
I've had a long hiatus, and am just getting back into things. I understand from what I've read that Grey Knights are generally considered very strong. What I'm trying to understand is what makes them this way. Is everything undercosted? Or is it just a few units that are crazy good, that get fielded by everyone, and skew people's viewpoint (and thus, if I limited the number I took, they GK army I fielded would be a bit more in line with the power of your average army out there).
To summarise my question, WHY are Grey Knights viewed as an extremely powerful army?
Once again, I'm sure this has come up, but I can't find an answer to this specific questions, so apologies if I'm digging up old topics.