My experience with the RT-6 is based on a a sample size of three, only one of which I personally own. Mine sits on a 18" rifle gassed build I use mostly for 2/3 Gun and Cola Warrior. I am not a serious
competitor, but I do ok all things considered.
The other optics belong to a friend and a guy I shoot against. We are all middle aged and spread of recreational funds among various interest. The goal in each case was getting maximum advantage for as little investment as possible. They both adopted the optic unseen. I only bought one after seeing theirs
The only other budget LPVO in my household is the MTAC on the son's carbine, but I make it a habit to shoot and compare optics with anyone that is interested, so I keep and idea of what is out there and how things stack up.
Optical Clarity: The RT-6 doesn't have the extreme fisheye commonly found in LPVO of its price range. In clarity, it is not a Vortex Viper or P4Xi but it is astonishingly close given the price difference. I think it may be the equal of the Burris XTR II line being phased out, but I need formal comparison before I call that one.
Reticle: The RT-6 is etched glass, which I like for durability, although it more difficult to get daylight bright. The P4Xi has a daylight bright dot, but projects onto aluminum wire
Great shortcut for daylight bright dot, but it makes for a weak point in an otherwise great optic. The RT-6 is not daylight bright in my book. It will wash out in bright enough sunlight.
The reticle itself is a horseshoe dot with BDC holdovers and wind holds. Only the horseshoe and central dot are illuminated. I think it is a great compromise although I might have done mils instead of BDC.
Eyebox: The RT-6 has better eyebox than the Strike Eagle. It is notably faster to get behind at hight magnification, although it isn't as pronounced with shooters that are well practiced with a Strike Eagle. The trade off is slight reduction of FOV. Physics can't be cheated. When say slight, I think the difference is like 9" at 100 yards on 6X. On the practical side, the outer 10% of the Strike Eagle's image is of limited usefulness, so you might actually lose ground. To beat the RT-6 eyebox you have to step up to the P4Xi and even then it is only an incremental step.
Durability: This is the real question. Fortunately the answer is easy. Durability in a LPVO of this price is highly questionable. To make a scope more robust takes additional $$$ or additional weight. You start to see the some benefits in $600-800 optics. The Burris XTR II is a good example.
As for my RT-6, it's seen fewer than 3k rounds and a few instances of indifferent handling. It hasn't died yet. I consider it and indeed all entry level LPVO to have the life expectancy of a goldfish won at the county fair. They are going to die on you and you'll never know when.
The most commonly broken optic in 2/3 Gun is the Strike Eagle. This isn't because there is some fault to it, it is just the most common entry level optic. It's that simple.
If you need a truly durable optic under $400 my choice is a basic Aimpoint. I think prism sights are generally more robust dollar for dollar than LPVO, so if you had to a magnified optic and real durability is an truly an issue, this would be my pick.
Final Assessment: If you are set on a LPVO and $400 is a hard limit, durability isn't really a thing. Glass quality and overall feature combination makes the RT-6 a winner with the next step up being the Steiner P4Xi. If you look back at the thread, I think you will find these are the two most recommended optics. There are reasons for this.