Did you find anything ?
I did some quick math, since it is hard to get data on Hive Engine. I want to query more users to get a better average of returns, but I do have something.
Short story
It looks like miners have about a 1.86 years break even period on STEM Miners. While this sounds like a lot, it is less than half the time of most tribe miners and you can always sell them at any time for near what you paid for them as there isn't many for sale at any given time.
Long story
I took a user with 77 STEM Miners, and took his last 24 hour mining. I wanted to get more samples before reporting back.
77 Miners @ 20 Hive each = 1,540 Hive up front.
This user mined 14 times in the last 24 hours of my test, at 0.9312 STEM a piece that is 13.0368 STEM.
13.0368 STEM is roughly 2.26775 Hive a day.
2.26775 Hive a day * 365 days/year comes out to 827.72875 Hive/year.
1,540 Hive up front costs / 827.72875 Hive/year comes out to 1.8605 Years.
(Update: This is now around 1.61 and shrinking as the Bernie sell wall is gone and the price is free to move).
A LEOM Miner for example makes around 1.68 Leo/year, which comes out to around 3.8 year break even, like the STEM Miners, you can sell them at any time for close to what you paid for them.
I just bought 44k stem FYI, what next ?
Well we know where Bernie's sell wall went now. :flex:
STEM works similar to Hive, if you stake it you can influence the reward pool based on your stake. We recently switched to linear rewards which gives everyone a fair chance at curation rewards. You do not have to worry about curation snipers affecting your curation rewards.
One our curation account, I did the math and was seeing as high as 179% APY on curation with linear rewards. I include the math behind this statement in this post.
The more stake you have, the more your APY will be as you can single handily get over the reward curve. There is a slight reward curve on author payouts, meaning lower payout posts don't receive as much per vote as larger payouts. This is to give a slight edge to well performing posts and make low quality posts with low rewards require more votes to start gaining STEM.
Author rewards burning
Not long ago we started burning 50% of author rewards not made on https://stemgeeks.net. This drastically reduced the rewards on users who are just adding our tags but not really participating in our community. This results in a 20%-32% weekly burn between the author rewards burning, admin accounts burning rewards, and promotion burning.
This means, there are very few STEM tokens available to be purchased and earned, far lower than most tribes. STEM already has one of the lowest inflation rates of all tribes at just under 7% (before burning), other tribes have as much as 19% inflation.
If you have any questions or need help, please join our Discord and ask any questions you may have.
Our biggest challenge right now, is to get more users on Hive posting about STEM topics and bringing in more users from outside of Hive to STEMGeeks. This is something I am actively working on.
There is a lot of information here, so I will likely turn this response into a post on 
RE: STEMGeeks burn report - 28.3% burned this week