The thing is, with a butt Joined end that has "Straight Screws" and just one Brace on one side, the opposite side is more likely to twist. The Brace helps of course....but the Brace would have to be a very skinny one if the Frame was built "on edge", akin to being a Picture Frame Corner Brace just 1/2" to 3/4" wide x 3" long on each end and of very thin gauge metal. Not a very sturdy, rigid support.
One aspect of a Keg Joined But Corner is the fact that the joining pressure is distributed at an angle, so the two flat joined ends must have far more force applied to twist or bend the joint out of true.
I've tried hybrid corners, employing Mitered Corners, Triangular Corner Braces, and alternating Screws...and with all that effort did come more strength....but also more difficulty maintaining a squared corner because the more angles and edges that are applied, any slight differences can become cumulative.
That also applies to any other corner. Get two corners just a tiny bit "off" (1/8" out of square) and you can wind up with a 3/4" difference diagonally, or 1/2" of a twist. Or more....depending upon the size of the Frame, and which side the error exists on.
Nothing beats well picked Lumer, exacting straoght cuts, and correctly joined Butt Corners for simplicity and sturdiness.
A cautionary tale.
Back in the day when I was a "Roller" man, I fought like 'ell against the inequity of telling people that to get really great results, they had to use a HVLP rig. That is until I saw the difference using one made. Then I could never consider anything else. Yeah, my apps do demand thinner coats, so there was that to consider...but consistency time after time was another benefit that could not be overlooked.
Same thing went for spending money for some Infomercial kinda gimmick like a Kreg Jig. I felt I could easily do without it. That is until I added up the expense of Braces...and re-dosand saw one at Home Depot for $99.00. I reasoned that if it actually worked as claimed, I'd make that money back in just 2-3 Frames! (...and repair some Furniture along the way...

)
What resulted was lighter weight frames, simpler corners, and straighter, truer frames that cost less.
Now everyone might not see things that way...some will insist that over-building a Frame is just as good. I once did.
But when one considers the few tools needed when using a Kreg Jig.
- Cordless drill
- Carpenter's Triangle
- Skill Saw
.....and that's it, then almost anyone can get'ter dun right, right out of the gate.