In any case, the paper I submitted is one I coauthored, so I like to think I'm a reasonably good judge of what's important. Maybe the tool just isn't a good fit for my field or my writing, but the highlights appear to be essentially random.
Thanks for reporting this! All feedback is really welcome. Firefox is supported. The error you encountered originates from the underlying PDF renderer which, depending on the browser you use, sometimes throws an uninterpretable error. It's on my list...
With respect to your highlighting results: I am aware that it can be a hit or miss at this stage. I've had really mixed feedback so far and I do have a theory that the quality of results may depend on the kind of writing (which is odd, bit I digress). If you could send your document to hello@tuchu.app that would be really useful!