Scala. The compiler is slow; sbt is buggy; too much syntactic sugar; implicits; cryptic; unreadable; and my biggest issue, symbols are reused and their use changes depending on how they are used, let's look at _:
As an existential type, as higher kind type parameter, as ignored variables, as ignored parameters, as ignored names of self types, as wildcard patterns, as wildcard imports, as hiding imports, for joining letters to punctuation, as assignment operators, as placeholder syntax, in partially applied functions, when converting call-by-name parameters to functions.