About 4-5 years ago, my MacBook Pro's trackpad would randomly stop working and I would have to restart my laptop a few times to get it working again. Sometimes it would work for a few hours, sometimes a few minutes. I also dual-booted Linux at the time and the funny thing is the trackpad never had an issue under Linux. After quite a bit of researching, it led me to believe that the ribbon cable connecting the trackpad to the motherboard was faulty due to macOS never trying to reconnect to the trackpad if it loses a connection for even just a moment.
I brought it to the Apple store and explained that I had tried everything possible to resolve the issue without opening up the laptop myself, such as resetting NVRAM, completely reinstalling macOS completely, etc. I explained that I think that the ribbon cable is simply faulty and that it probably just needed a replacement. The Genius said that they're going to bring it in the back to do every single one of the things I mentioned that I already did.
I come back 4 hours later to pickup my laptop and they tell me that the issue is the motherboard and that replacing said board would cost $800. I asked how they determined it was the motherboard and not simply a faulty ribbon cable to the trackpad, but they wouldn't answer the question directly. After requesting their superior and restating my question a few more times, they told me that they don't have access to replacement ribbon cables and that my only option was a motherboard replacement, which itself would take about a week because they needed to order the board. That was enough for me to take my laptop and leave, even though they still didn't answer the actual question of how they determined what the actual issue was or even confirm my hunch that the issue was with the ribbon cable.
I bought a $5 replacement cable from Amazon, took about 10 minutes to swap it out using an iFixIt tutorial, and I'm still using that laptop as my main machine years later. In other words, they spent 4 hours to quote me an $800 fix that would take about a week to be done for something that was actually only a $5 fix that would take no more than 30 minutes. I've been hesitating to buy a new laptop for years because of that experience as well as the downhill quality of MacBooks as of late, such as the keyboard (although fixed now), touch bar, initial Catalina bugginess, etc.