I think this is the aspect of modern approaches async which I am most ambivalent about. One of the things I have learned about programming over the past 10 years is that I, as the programmer, really want to own the flow of control of my program. Once I hand that over to some other system, usually in the name of convenience, I will start to have issues which are difficult to understand and solve.

For instance, a while ago, I was working on a project which was using making heavy use of RXJava. One of my colleagues pushed a commit, and suddenly CI was failing on a unit test which passed when run locally. It turns out the problem was because the CI server was running tests with a different scheduler, so GC was happening at a different time, creating an NPE which didn't happen locally. Imo when you start to see unit tests behaving inconsistently based on a factor which is completely outside the actual code you yourself have written, this is a sign you are going down the wrong path.

I also wonder how much a lot of the buzz around async actually has to do with the fact that it's a bit brain-bending to wrap your head around at first, as compared to its actual utility. I think for a lot of us as programmers, we enjoy that feeling of understanding something difficult - like when you really get recursion for the first time - and we're attracted to the idea of really fundamentally new concepts being introduced to programming.

But it seems to me that async is one of those concepts which brings us farther away from actually programming the hardware, and puts a kind of middle-man between us and the CPU, and I'm not sure that has ever been a good thing.