I've got two Road Morphs on the seat stays of two different bikes. No worries for me! Much easier to transport, too.

Note both of these have the newer clip on and velcro attachments. I think my older one (slide in and clip) has come loose once in, oh, 8 of 10 years, probably because I didn't slide it all the way in.

I've never really noticed a lot of dirt, even though one of the seat-stay mounted pumps is on my rain bike. It's on the forward side of the stay, and of course fenders reduce spray from the front wheel.