Kevtris explained the power draw of the SuperNT over on Atariage (he answers questions in his old FPGA thread there).
http://atariage.com/forums/topic/242970-fpga-based-videogame-system/page-263#entry3958632
The official adapter is 2A. Imagine they just decided to err on the side of caution a bit. Using a lower rated adapter is more likely to cause problems with the adapter than anything else (pushing it by pulling more than it is rated for is hard on power supplies and can burn them out faster). Although someone said that they were a little under-amped over on Atariage and they were getting audio skipping and stuff.
Just make sure to use a good USB cable / adapter - scrounge one that Samsung or Apple or LG or someone else reputable shipped with a phone or some other high-end product that is likely to have gotten good QA. Something putting out 1A - 2A. If you can't remember which is which, check the label - sometimes the part number is on there and you can google it and find what device it came from.
Cheap cables I think are more dangerous in the USB3 world (more sub-par cables out there than USB2) but cheaply manufactured stuff is always dangerous.
I'm using one of these usb charging gadgets for powering / charging a few things on my AV stand. I've used one for years for charging phones/controllers/gadgets in my desk drawer. Use it to power my Retropie and anything else on the AV stand that needs usb power or charging.
https://www.amazon.com/gp/product/B00UN1LM7Q/
Thank you, this was super helpful :)