I'm at almost fifty years of programming (APL was an early highlight; punched cards weren't) in many languages. Haskell isn't the be all / end all of languages, but when I want to add ten lines to a program and have it use every core, I don't know a better choice than Haskell. Nothing else comes close.
The choice of the GHC Haskell to choose on-use evaluation as their non-strict evaluation strategy means that it cannot simply evaluate all function arguments in parallel. On-use means that if you do not use it it does not get evaluated, which some code relies on. Plus, as the trope goes, Haskell is implemented in a very imperative way, which can lead to some fun surprises when parallelizing aggressively.
There are like three different flavors of parallel programming in Haskell, differing in how much you want to trade off performance for having to know about compilation, memory management and execution details to make it run efficiently in non-trivial parallelism cases.
The main issue I found was the language and libraries are really hard to pick up as a beginner. A library for ruby or python would start off with a few examples showing how the most common use case works and then full api docs for the library. Haskell libraries usually only provide a function name and type definition and if you are lucky you get a one liner telling you what the function does but not how to use it.
This seems to be not an issue for haskell experts because "PathMultiPiece ps => (ps -> Writer LiteApp ()) -> Writer LiteApp ()" is all the info needed but I found it much harder to use because of that.
I also found that many of the libraries were totally inaccessible unless you had an understanding abstract mathematical constructs. I wanted to do some basic xml parsing in haskell and the prerequisite was reading some huge book on the theory of some math behind how the library works. I then switched to ruby and nokogiri had on the website basically the exact code snipit I needed.