In my opinion, he has some good music, but a lot of it isn't even good enough to gather dust.
what is that supposed to mean? can you give an example of some of his works that "isn't even good enough to gather dust"? the only work by him that I have heard that was mediocre was the wind octet that was not even that bad.
I've heard all of the piano concertos and some of the piano works. There were some nice moments in the first piano concertos, but the others seemed awfully generic and didn't make much of an impression on me. I felt the same way about the piano works that I've heard.
As far as I am aware of he was the first one to: write a piece for a single hand, first to use out of the way instrumentation (piano, horn, oboe) first to have a set of variations on a non-original theme in a sonata. He was also an important teacher, some of his notable students were Grieg, Albeniz and Janacek.
All those composers lived 100-200 years ago, but are still played.I think modern contemporary composers are underplayed/underrated
Some of that stuff can be argued. Carl Czerny wrote a piece for a single hand before Reinecke, actually (an etude). Also, being an important teacher doesn't make you an important or a good composer. Just look at Paul Dukas (the sonata is probably his only great work).
I don't mean to keep spamming with underrated composer, but the Austrian composer Apostel deserves much more recognition.