I wouldn't trust it. especially thinking a child could be climbing on it. I would be to afraid it could give way. or even a adult leaning on it. I rather have screws holding them together.

But I know nothing about carpentry this is just my common sense talking

Here are some of my concerns off the top of my head

1. Screws driven "straight" into end-grain, = weak connection
2. Top screw on rail connection too close to top of rail, potential splitting leaving all pressures on single (bottom) screw
3. Both he rail and post connection screw holes are in-line, should be staggered and/or alternately angled to avoid splitting
4. No ratings, codes, or regional compliance's mentioned, (is it legal) let alone safe.
5."durable polycarbonate" is vaguely descriptive, unknown strength or longevity.
6. Longevity, will it become brittle due to climate, most polycarbonate things will at some point.
7. Expense

If this was marketed as a K-D type of connection, intended for temporary use I wouldn't see a problem. There is however, no mention of this and it "seems" to be an alternative to traditional post and rail construction.

these are made for those who are too lazy to do a good job and want it done fast.
"look dude, i built a deck"