Interestingly, I agree with what you say about his "chop & change" approach (it isn't really, but I can't think of any better way of putting it at the mo) to the majority of Haydn Sonatas, especially in his first movements, but I find that is one of the charm he offers... You open the door which you think will lead to a corridor, but actually, it was a grand drawing room... kind of a magic or surprise element in his structure...
I am fond of Haydn Sonatas (ok, I only play around 6 or 7), particularly his first and second movements. Third movements in general, is a little "light" for me... In some ways, I prefer Haydn Sonatas rather than Mozart's, even though Mozart IS my favourite composer... Mozart's Piano Concertos, these I love...