The trouble with AS is all about existing implementations, really... Say, the way they try to optimise the language.
this scope is slow in ABC, but don't know in AVM2 evaluation, though.
One limitation too is that most free hosting services only support a internal PHP system, and don't allow accessing that internal, so one is only able to run in an limited PHP environment. But I don't understand why Atelier 801 uses Java/JRE in server-side, but Flash and AIR only in user-side. They could use RedTamarin to have AS3 in all sides. Some other people use Python and a compiler or in-fly evaluator, but that isn't a good idea. AS3, for example, is always better.
It's also sad AS3 prefers UCS-2 (+ UTF-16) encoding over UTF-8, and that's due to ECMA-262......