I feel more comfortable drawing characters, because I have more practise drawing them. However, I am slowly gaining confidence in drawing environments, since I'm now drawing them all the time.
Drawing the environments your characters are in helps establish a sense of place. It helps the reader feel that the characters exist in a context, rather than floating in a vague void. The vague void works perfectly well for gag-strips and certain varieties of slice-of-life comics, but if you're doing something more along the lines of adventure or mystery or action, you're going to want people to feel like these places are well-defined and certain, or else they might get confused.
As for how to avoid artist block.... Just keep drawing, even if it sucks. Avoiding drawing something isn't going to a.) make it easier, b.) improve your skills at it, or c.) make it magically appear on the page. The only way out is through.