How many reels do ya’ll see in a day? I’ve spoken to general game recruiters and I’ve heard of about hundred(s)+ reels a day, but that’s in all of the disciplines, not just VFX. I’m still working on my first real VFX only reel (ugh… English), so when there is a topic talking about VFX demo reels, I take note. For this one though… well…
To quote my inner student; “Aw man. These are all so varied.” 
(ಥ﹏ಥ) I just want to make virtual things blow up nice.
I would like to give MY take on all of this. MY observation based mostly on what has been posted here at this point, keeping in mind - yes, of course context. It seems to me right now that the best thing to do would be, as a lone artist, to start the demo reel showing off JUST the VFX in question spun in a round table if possible (cards being what they are). Then show it off in a grey boxed normal mapped lil’space. Something simple and singulair where the effect can be shown as little more dynamic then just - hey look at my effect spawning infinitely in the void. After that the only issue becomes how do I time all this to be quick and sweet in the reel.
I know ya’ll don’t have all day to watch my feeble mortal attempts at magic. 
Anywho, part of my reasoning in all this, I should note, comes from outside this thread. I have searched for many a VFX demo reel. Most demo reels I have seen involve game play utilizing the VFX in game - Where it matters! So there is a part of me that wants to “monkey see - monkey do” and show off effects in a space where they would live, even though that means I have to put some time into making-assets-of-whatever-complexity-not-VFX for the VFX. Stuff like a teepee style log fire space or ancient egyptian alien staff for a laser effect. Something to plus the existing VFX animation into because, simply put it’s… nice. And we’re artists… eh… of some-sort anyway. In my case, while I don’t necessarily have a game to put stuff into, I can show an effect among objects or a simple environment space. So… all in all, I figure go for the happy medium, and be careful of feature-creep, as always. Just don’t district the purpose of the reel - and you’ll be (I’ll be?) gold.
( * Psst * What do ya’ll think of my observation? Should I do this? 
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(2 Minute Edit: Apparently while I was making this post, this thread was made as a continuance. I’m going to go read it now!)