We will be adding a full settings page for Custom Bangs, which will include some more advanced encoding options.
In our experience & from bug reports over the years, some services are indeed sensitive to encoded vs not encoded, and other details (%20 vs +). We have a suite of overrides for known hosts.
A quick check on DDG, they do some encoding as well. They likely have a similar set of special rules to make popular ones work correctly.
