[I]Tl;dr version[/I]: don’t worry about stuff breaking unless it’s a manufacturing flaw, and Sennheiser should be more than fine for you.
If I recount the experiences others have had:
For headphones with remote controls, the malfunction may cause the player to raise/lower volume on its own, or randomly switch songs/pause playback, or just causes the remote to not work.
For the inline volume control that doesn’t rely on the player (where you set the player to the maximum volume), I think it would simply cause failure to produce sound in one or both channels of audio.
How likely this is to happen depends on your intent to be active while wearing the earbuds/headphones. I have had a remote (on a much older Sony CD player) get glitchy after getting wet/dirty, but it still works. In another instance the original ear buds that came with my iPod shuffle (where you needed the remote to be on the earbuds to have any control) have yet to be affected by anything & still work fine.
Your mileage may vary. I tend to have issues with the strain relief at the headphone/earbud jack. Mostly the strain relief shield detaches from the plug & can slide freely along the rest of the cable (super glue fixes this), though on my oldest pair, I failed to get to it fast enough (so under an extreme bend, the sound may cut out).
I believe if you are cautious with your headphones (read: don’t abuse them, but don’t treat them like a fragile egg), you can usually avoid a major malfunction for quite some time.
And yes, it really IS difficult to understand how well things will fit without personally testing them. I found that my ears genuinely feel most comfortable with the Sony fit; I expected the Denon and a cheaper pair of Koss buds to fit the same, but they were more rigid and slightly larger (in addition to the Denon being air-tight). Luckily, the Klipsch are quite narrow and flexible, so I feel safe going with them again in the future.
Since Sennheiser has the many tips to get a better fit, and since I have only heard good things about them, I’d say you would be safe going with Sennheiser.