From experience, I have used a TSI when I need help with a bug in Apple's products. Every single time (about 5x) they "refunded" the TSI because it was an Apple bug. I have never had to pay for extra TSIs -- I use the ones that come with my developer account and (so far) they are always refunded. I don't know why people hoard them -- they are use it or lose it every year.

I have also gone to a virtual dev lab during WWDC which was awesome. Lots of "free" help from an Apple engineer (basically 30 minute speed run of lot of questions about Apple Watch workout app development).

I have mixed experience with Feedbacks. I have filed dozens that I actually care about and maybe 10% were a decent experience. The rest were essentially ignored.

I filed a TSI earlier this year. It look them six months to reply, and even then it was to ask for information that had already been given in the original report. By then I had already figured out what was causing my problem and found a workaround. Still, I thanked the developer for their time, described my approach and asked if there were a less hacky way to do it. I never got a reply.