I don't think Pawn would be hard. I think that there are tasks that you can't achieve in pawn because of its limitations.
Besides that, eg. you can't simply concatenate a string (face it, this is a
very basic task). You can't simply use an associated array, you should use Tries instead, which are much complicated (for a very basic task, again).
For extending a HLDS functionality, you have to learn a new language which you can't use anywhere else.
You can't do clean code in pawn.
You can't do OOP in pawn.
JavaScript has proven. It is actively developed, standardized and the V8 engine is maintenanced by one of the biggest IT company in the world. Just compare release times, Pawn's last release date is 2012-06-22, the AMXX compiler uses a 2006 dated version.
Should i continue?
Anyways, i don't want to "rule the world" or something, i just want to use JS for extending HLDS. Since it hasn't done before, i should do it (though i can't code so good in c++, so the first version will be deterrent
).