Bad compile-time performance. Due to the way flattening works in Ether and due to a GHC bug, the compile-time performance was devastating. Turning -O2 could mean hours of compilation and required up to 65 GIGABYTES of RAM (ridiculous!). This was because GHC generated an exponential amount of coercions (as evidenced by investigating .hi-files). Basically, @int-index no longer can recommend Ether to people as there is no good solution to this known.