I'm pretty new to the industry so it's early days for me.  I'm looking to put together a demo reel of music to market myself as a composer/producer of game music.

Does anyone have any advice on optomising a game music demo reel to cover key themes and functions to best make it relevant to developers in today's market?

Should I look to cover a niche (I tend to create pretty abstract stuff that probably best suits sci-fi), or try to cover as broad a range as possible?

I think you'll be better off in the Music & Sound forum for this one, I'll move your topic across.

You are probably best served covering the style(s) you do best. For example, if you're not that great at reggae and put up some mediocre reggae music, then it's not going to serve you or potential clients as well. But if you put up music that you're most passionate about, that always shows off your best skills, then people will react stronger to that and you'll get better interest in your work. 

The only other thing I'll add is make your demo reel easy and quick to navigate. Nothing's worse than having to muddle through long demos to find the style(s) or genre(s) you, as a client, want fulfilled.