I have both types of keyboards (pre-2016) and the 2016 Touch Bar 15" MacBook.
In my experience there are about three failure modes, some of which you can fix by yourself. However, since the keyboard replacement program started I just stopped trying to fix them myself and instead brought them in, because they give you a free new battery too.
1) The key gets physically stuck up and won't press down. You can fix this sometimes by just smushing down the key and crunching whatever crumb was in there down.
2) The key doesn't respond or needs to be pressed hard to respond. This one is hard to fix, but what I tend to try is to get some 70% isopropyl alcohol and a q-tip. Remove the keycap, and dab repeatedly hard and fast with an alcohol soaked q-tip on the round button dot thing. The idea is to get the alcohol in under the dome, and have it dissolve/pump out whatever was stuck in there. A few q-tips worth of alcohol shouldn't reach the logic board, but don't pour alcohol on there directly. When in doubt blow on it, let it dry out, and try again.
3) The key repeats. This one in my experience is easily fixable with the above alcohol and q-tips method.
I've used these methods on a 2016 13" touch bar (problem 2) and a 2016 15" touch bar (problem 3; put in little effort and gave up and elected to get a free new battery). Both were purchased used for cheap, and the 15" touch bar had previous coffee in the keyboard, as I later found out (a little.), and it surprisingly worked fine for about a year until it became unhappy. I've also had a 12 inch MacBook from 2016 that was just fine, even after eating crumby food while using it on vacation.
Based on these experiences, I don't think the new keyboard design rolled out in 2018 with the silicone membrane solves anything but 1), the least frustrating of the three.
I haven't had a dead keyboard to destructively tear down yet, but from observation it appears that the keyboard is made as a plastic sheet with traces on it, and a little metal dome that pops up and down to make or break contact with the traces. Over the domes, there's another cover sheet that's not completely sealed, because it has cut outs for the key hinges and a little weep hole for air to come out when it's pressed. I think the dirt is going in through these. Perhaps if they move to capacitive sensing, or add a little nub under the metal dome and replace the switch layer with the pressure-sensitive sandwich from the older kinds of keyboards, making it so that the part that sucks dirt in doesn't have to make contact but just pressure, this would be resolved better.
The thing is, when typing I really do enjoy the newer type a lot more since the lower key travel requires much less force to actuate than the older ones. The actuation point is also much more precise due to the metal domes vs the rubber ones in the older ones.
In my experience there are about three failure modes, some of which you can fix by yourself. However, since the keyboard replacement program started I just stopped trying to fix them myself and instead brought them in, because they give you a free new battery too.
1) The key gets physically stuck up and won't press down. You can fix this sometimes by just smushing down the key and crunching whatever crumb was in there down.
2) The key doesn't respond or needs to be pressed hard to respond. This one is hard to fix, but what I tend to try is to get some 70% isopropyl alcohol and a q-tip. Remove the keycap, and dab repeatedly hard and fast with an alcohol soaked q-tip on the round button dot thing. The idea is to get the alcohol in under the dome, and have it dissolve/pump out whatever was stuck in there. A few q-tips worth of alcohol shouldn't reach the logic board, but don't pour alcohol on there directly. When in doubt blow on it, let it dry out, and try again.
3) The key repeats. This one in my experience is easily fixable with the above alcohol and q-tips method.
I've used these methods on a 2016 13" touch bar (problem 2) and a 2016 15" touch bar (problem 3; put in little effort and gave up and elected to get a free new battery). Both were purchased used for cheap, and the 15" touch bar had previous coffee in the keyboard, as I later found out (a little.), and it surprisingly worked fine for about a year until it became unhappy. I've also had a 12 inch MacBook from 2016 that was just fine, even after eating crumby food while using it on vacation.
Based on these experiences, I don't think the new keyboard design rolled out in 2018 with the silicone membrane solves anything but 1), the least frustrating of the three.
I haven't had a dead keyboard to destructively tear down yet, but from observation it appears that the keyboard is made as a plastic sheet with traces on it, and a little metal dome that pops up and down to make or break contact with the traces. Over the domes, there's another cover sheet that's not completely sealed, because it has cut outs for the key hinges and a little weep hole for air to come out when it's pressed. I think the dirt is going in through these. Perhaps if they move to capacitive sensing, or add a little nub under the metal dome and replace the switch layer with the pressure-sensitive sandwich from the older kinds of keyboards, making it so that the part that sucks dirt in doesn't have to make contact but just pressure, this would be resolved better.
The thing is, when typing I really do enjoy the newer type a lot more since the lower key travel requires much less force to actuate than the older ones. The actuation point is also much more precise due to the metal domes vs the rubber ones in the older ones.