(Update at bottom)
Earlier tonight, on my first working use of SteemPress, I discovered that the plugin developers 
Setting aside the absurdly huge cut they're taking, the more disturbing aspect is they have not mentioned this ten percent haircut anywhere that I've been able to find. It's not mentioned in their posts on Steemit, the SteemPress plugin information, or the github repo. Go look for yourself, if you'd like:
Steemit Posts
Introducing SteemPress beta, a wordpress plugin for steem
Steempress developer update #1---(Archived)
Third Party Sites
The SteemPress plugin page at Wordpress---(Archived)
The Github repository---(Archived)
Nor do I find any reference to a ten percent cut doing various searches through DuckDuckGo. I think I've done a sufficiently thorough search to conclude that it simply is not mentioned anywhere, not even in the plugin code if you want to dig through that.
Instead, I first found out about it when I made a test post through the plugin and saw something new pop up on my steemd page:
I watch my account on Steemd because it's easier to see upvotes and comments there, and I'd never seen that note before. Beneficiaries? I've never set anyone as a beneficiary of my posts. I have no reason to. Didn't even know I could.
So what does that mean? The total weight of rewards in a Steemit post, as Steem currently works, is 10,000. That's the number set in the Steem config file if you want to dig it up yourself. So each 100 points is 1% of the reward. Each of these developers is getting 500 points of the reward weight, or 5% each. See below:
You can see the reward_weight at the top of the image, that's the 10,000 total. And you can see the 500 each developer has gifted himself from my work. The posts I've submitted through Steemit alone look like this:
As you can see, the beneficiaries array is empty on a normal post. Because, again, I've never set beneficiaries and I didn't even know it was an option until today.
You can see the information about the post I posted through SteemPress here, and an example of a normal post here. Just click on the "advanced mode" link on the top right to expand the data.
But maybe it's something I can turn off, or adjust. Maybe it's some option that's set in the SteemPress settings? Nope, there's no option to do that:
What about in the plugin code? Is it some careless remnant from development that was left turned on when the code shipped? I spent some time digging through the plugin code and it's clearly not a setting leftover in there. No, the beneficiaries are being set on a black box server controlled by the developers.
It's clear the problem is as I originally stated. The SteemPress devs have cut themselves in on the rewards for the posts made through their plugin, and you have no choice in the matter. Nor did they ever mention anything about taking a ten percent cut in any of their posts or documentation.
This is incredibly dishonest.
Whatever you might think about third party developers taking a (huge) cut of your rewards for using their service, they absolutely have to be up front about it. And the SteemPress developers have failed horribly in that respect. They have asked you to trust them with your private key, and immediately betrayed that trust.
Further, without being able to look at their black box code, we have no idea if they're even handling the keys securely. Your private posting key is very important, and should be well protected. Even if someone can't steal your money with it directly, they can flood your account with spam posts, upvote whatever they want, and generally wreak havoc. Do you want someone posting pornography or gore on your family-friendly account, or worse, posting illegal content?
And will
Given the dishonesty so far, I'm not willing to take the chance by continuing to use their service even if they dramatically reduce their cut and start being honest. I would've probably been okay with 1% total if they were up front about it, but that ship has sailed.
I already changed my keys and uninstalled SteemPress. It's up to you whether you want to do the same.
UPDATE:
The SteemPress developers put out a new post a few hours ago (1:15 PM EST, Jan 5, 2018) where they finally mentioned that they were taking a cut. You can see it here:
SteemPress now available in the WordPress.org plugin store! – Introducing V1.0
My opinion has not changed. You get one chance to be truly honest, and if you only decide to tell the truth after someone calls you on it, you don't get credit from me. Especially if you don't even acknowledge your screw up.