Buddhists can also be very good friends. It depends on our expectations and the individual processes our friends are going through.
Having in mind the context surrounding friendship among Buddhists, one is expected to understand if a friend leaves for retreat. Not having said anything in advance may be due to a lot o reasons, but we can surely understand the benefits of retreat. If a Buddhist friend of mine goes for retreat, I would be very happy, not mattering if he doesn't return my phone calls.
It's also natural that "hanging out" with them doesn't feel the same. And it's natural that now and then they go through tough phases, as meditation does that to you on occasion. Some deal with it while keeping a balanced social life while others do not, needing some time for themselves. As friends, as Buddhist friends, we should understand it.
My experience is a bit more like enjoying it a lot when we are together and when we part ways we don't leave craving for the next time. A phone call or an email now and then and that's enough. When possible, we visit. Sometimes weekly, sometimes monthly, sometimes years pass. The friendship remains.
We usually approach relations in a very needy way. This is what usually doesn't work so well with people engaged in serious practice.
Dharma centers are a different issue altogether. When I speak about Buddhist friends, I mean people I know well, practitioners, people who have at least a good grasp of the basics and are engaged in practice. Dharma centers, sometimes and not all, are filled with all sorts of strange people, that I would hardly call Buddhists. There are Buddhists there, of course, a small minority. But then you also have the usual poseurs, all sorts of freaks who try to adapt Dharma to their views, people with serious psychological issues dealt in the wrong way, all_religion_is_the_same sort of guys and what not. Now, it doesn't really matter where one comes from if one is going to practice the Dharma. But this assumes that one will indeed practice the Dharma, instead of just adding a shinny Buddhist layer to one's ego, or just spending the time gossiping, or trying others to accept personal corruptions of the teachings and so on and so forth.
It was Dzongsar Khyentse who said that we need a healthy ego to destroy our attachment to it? I think so, but if it wasn't him, it was some other reputable lama.
And we must remember that in the West most balanced and well adjusted people don't become Buddhists. Why would they? They feel they have achieved most of what they wanted in life.
It's a bit like physics students in Portugal. You have the few ones who choose to graduate in physics and the ones who ended up there because you don't need high grades. Then you have few terrific students and a mass of lousy students, with little in the middle. Most Buddhists I've met were either a little dumb or pretty much above the average.
Many had serious issues, while others were very balanced, usually the ones who ended up engaging in practice. We don't see many Namdrols in our sanghas. Or Pema Rigdzins, Anders, Astus, Husengs and so on. There are many others around, but you get my point.
Perhaps this is why these "Buddhists" you talked about don't make very good friends. They wouldn't make good friends if they weren't Buddhists all the same, because they're people who have their heads all messed up. And I don't know if becoming Buddhists will make it better...