I'm thinking about the products I've seen them on now and it's always the more ruggedized boards but not the ones that are so ruggedized that they get potted or placed in hermitically sealed housings. Maybe if it's potted or sealed well enough you don't need the added environmental resistance and stability offered by a MELF.

I've never actually worked on anything that went into space to my knowledge but I imagine most electronics going into space you would want to be potted or sealed inside something just so the electronics have something to disapate heat into. That's about the only situation I can imagine where potting would actually improve thermal characteristics. If that is the case then they would probably not use MELFs for the same reason the sealed electronics I work on probably don't. Of course I'm still guessing on all of this though.