Without additional information, I would assume the same current rating as every other pin. So, probably 10 or 20mA is about as high as it should go. 15 is in that range, so, probably OK.
The other thing that's a pain, is EMI. This is true regardless of how many bondwires they've used: bypassing can't be any better than the two pins, a single bypass cap (or however many you try and stack together), the pin lengths, and the return path. Every CPU cycle, every pin toggle, can be "heard" through every other output pin. Definitely a case to avoid routing GPIOs (particularly outputs) to the outside world. They'll probably be dirty in the 10s of mV range. At the very least, filter such signals before going out to an unshielded cable (and, needless to say, whatever ESD protection that might also want to have).
That's not usually a problem (use proper interfaces where possible e.g. RS-232, 422/485, etc.), but it can be a concern in that intermediate space where you have long traces running across a wide board, or short interconnects going to, say, an HMI board, or panel lighting/indication -- LEDs on pigtails? -- that sort of thing.
Tim