Answer:
They should be used sparingly throughout the essay. If something is important enough to be in the sentence, it should be fully part of it. As always, make sure to check with your teacher or professor.
Hope this helps!
If you are telling us to revise the sentence, one error is that there is a double negative. 'not' and 'un' would cancel each other, and the revised sentence 'I wouldn't say he's the nicest guy around, but I've seen him behave kindly before.'
