Are you quite certain you called Apple and not some scam outfit fraudulently representing themselves as Apple?
Perhaps your Time Capsule is not reliable, in which case it should be repaired or replaced, but I doubt that's the case. To characterize the Time Capsule as an unreliable product is simply not a credible answer, and it should be summarily disregarded.
The only reasonable explanation is to characterize wireless communications as generally unreliable, which it can certainly be, due to the vagaries of that communications medium. To identify potential wireless communication problems so that you can explore possible solutions to them, please read Check for Wi-Fi issues using your Mac - Apple Support. It's very informative. So far, you have been misinformed.
As I wrote there are additional help resources, such as the following: If a Time Machine backup takes longer than you expect - Apple Support. There are many factors involved. I suggest you review all of them.