> every dynamically-typed language has "weird" behaviour around type coercion

Do they now?

    $ irb
    irb(main):001:0> "0" + 1
    TypeError: no implicit conversion of Fixnum into String
            from (irb):1:in `+'
            from (irb):1
            from /usr/bin/irb:11:in `<main>'
    irb(main):002:0> ^D
    $ python
    >>> "0" + 1
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: cannot concatenate 'str' and 'int' objects
    >>> ^D
    $ node
    > "0" + 1
    '01'

> Meanwhile, those of us using JavaScript simply run one process per core and never worry about contention, deadlocks, or race conditions.

If you never worry about those things, then you're either not sharing any resources at all (in which case threads are also dead easy), or you're failing to deal with the shared resources properly.