Singletons are easy to understand, as long as they contain of one simple class. But after a few iterations of development, they tend to "capture" a lot of dependencies, which practically become singletons too. A lot of mistakes happen. And most of the time, there was no good reason to create a singleton in the first place.

see also those posts: https://stackoverflow.com/a/138012/4249619 https://stackoverflow.com/a/142450/4249619