On IRC we get a steady stream of confused users (in both the #emacs and #clojure channels) who added melpa because some library recommended it in their readme and they didn't realize it would cause every package they use to also pull from the unstable branch. It is a huge support headache.
However, these days I only very rarely find packages that aren't available on Marmalade. When I do, it's usually enough to ask the maintainer nicely and they start publishing to Marmalade; it's not a difficult process by any means.
But one of the issues with Marmalade is that anybody can fork your project (patch it or not) and just push it. With MELPA, they try to get the original authors to submit the projects. If you've got patches, they are pretty strict about it and encourage you to get them merged in the package rather than accepting a fork.
Hmm. Is there a particular repeated source of breakage? I only use MELPA, and apart from the recent nrepl->cider migration (which was intentionally piloted on MELPA) I've not had any instances where my work was hindered by package issues. And as a maintainer, I update my packages daily.
However, these days I only very rarely find packages that aren't available on Marmalade. When I do, it's usually enough to ask the maintainer nicely and they start publishing to Marmalade; it's not a difficult process by any means.