I assume you mean the "FS5" flat switch.
The problem I've had with all such switches (although I've not had the Yamaha so might be talking out of my **** here) is that the switch inside is usually a keyboard switch which relies on the mechanical design and integrity of the surrounding unit to prevent the switch being overstressed and then breaking. If the housing gets warped / bent / bashed such that the switch starts getting mechanically abused it will fail very quickly.
Of course, I could be heavy footed....
But after breaking two "generic" flat-style footswitches in 2 months, I bit the bullet and purchased a Roland DP6 (similar to the Yamaha FC4 in design). It uses a microswitch internally and is built like a brick external tool storage facility. I had to rewire it work with my Ensoniq (but then they've always been different...)
I've had it for 5 years now, used every weekend. No sign of any mechanical or electrical deterioration other than external scuff marks. If the switch should fail then it's a standard type of microswitch so I'm sure I could fix it. If the unit breaks mechanically I would (a) be very surprised and (b) buy another one without question.
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John Allcock