The downsides are:

1. It’s slow in number crunching, if you’re doing any computation it won’t be much faster than Ruby (and much slower than Python due to the scientific computing ecosystem) 2. The amount of libraries is negligible, many are abandoned, even on its home turf (networked applications). I.e. there is no maintained ssh server lib, whereas all popular languages have several. 3. To get the benefits of the platform, you need to deeply understand OTP. This is a serious learning curve. 4. Despite all the chest-beating about performance of Phoenix, it’s solidly mediocre on any benchmark (about on par with the fastest Python web frameworks). It’s okay, but nothing to write home about.