Too many configuration options
It is a pity I have to search the web to find out how to get white text on black background (or black text on white background). Moon Reader seems to suffer from too many fancy new features. Too many configuration options, which users find difficult to understand, are an indication for that. I prefer no-nonsense tools.
It fits into this context that to fill out pdf forms, a pdf reader would be the more appropriate app than an e-book reader.