Sentiment analysis for the purpose of categorizing reddit comments, sure. For the purpose of giving you advice about nuance, overall clarity and tone of own long test, no.

I tried it myself, and it did actually a good job.

There's nothing magical about a long text you write yourself vs a stream o reddit comments in a thread. It's all sentiment analysis on text. It can extract ambiguity, how ideas are connected in the context, categorize and summarize, etc.

You should try it and see it for yourself. Feed it some large text of a single author and ask it to do those things, see if the results are satisfactory.