It's public knowledge that Apple added silicone membranes to its third-generation MacBook keyboards to prevent debris from getting in, but how well does that work, really? Not necessarily as much as Apple would like. In response to a Wall Street Journal piece highlighting ongoing reports of problems, an Apple spokesman said the company was aware of a "small number of users" whose butterfly keyboards were having issues, and that it was "sorry" for the problems. It didn't say this was a systemic flaw, however -- it contended that the "vast majority" of MacBook Air and Pro users were just fine, and that customers should contact support if there's trouble.
Companies like iFixit and Simple Mac might disagree. They believe the butterfly keyboard system (which allows for very thin yet stable keys) is inherently fragile, and that even the slightest amount of debris can jam the mechanism. It's also possible that weak springs render them prone to breaking down. And while any system with a third-gen keyboard is still covered under the free warranty, it's still a pain to wait for a repair for a seemingly chronic issue like this -- especially if you depend on a MacBook for your livelihood. There's no guarantee Apple will institute a special repair program, either.
While it's unclear just how many people are affected, the problem is sufficiently widespread enough that some are calling for Apple to ditch butterfly keyboards entirely. They believe Apple compromised reliability (not to mention tactile feel) in the name of a thinner design, and don't expect the situation to improve until Apple rethinks its strategy.