Mansfield Park is only a somewhat difficult book to read. Austen's language is a bit old-fashioned and the way certain sentences are constructed takes some time to get used to. The syntax, or word order of sentences, can seem a bit like Yoda-speak at times. Other than that, though, the vocabulary isn't overly difficult and the plot and ideas of the book are not that hard to follow.
The main difficulty in reading Mansfield Park actually has more to do with characters and content than language and vocabulary. Mansfield Park features a lot of main characters who can become downright frustrating at times, which can sometimes make it a bit hard to hang in there. However, these characters are also consistently interesting, even when they aren't very likable. The plot does sometimes drag a bit, but it picks up pretty quickly after the down-times, and is most often pretty fast-paced and entertaining. Overall, this book can be challenging but it is also a rewarding read.