While there are some easier pieces within Brahms Op.118, I think Brahms Op.118, as a whole, is harder than Schubert Sonata and Grieg Sonata. It's also more difficult to make Brahms Op.118 sound good and interesting.
It's worth remembering that Brahms's sets of piano pieces can be mixed & matched, unlike sonatas, where the composer conceived the movements as part of the whole. In fact, there is currently an excellent ongoing recording project where the pianist makes his own selection of pieces from Op.10, Op.76, Op.79 and Op.116 -119 in his own order to fill up each CD, and it works very well, shining new light on his late piano music in particular when heard in a different juxtaposition from the published sets.
In other words, one wouldn't normally play Schubert's D664 second movement as a standalone piece (nor indeed Grieg's Sonata slow movement), but the famous Op.118/2 is very frequently played by itself - by both concert pianists as well as amateurs. Many students only ever learn No.2, in fact. There is no reason why anyone should feel obliged to learn all the pieces from Op.118 (or indeed, Op.117 - of which No.1 is by far the most popular too) for performance purposes.
Whereas with a piano sonata, one wouldn't normally hear a concert pianist play a single movement, except as an encore.
As for styles, it is good to become familiar with different composers' piano writing, which is a very good reason for someone who's never tackled Brahms before to play him.......