Those shelf standards with dual rows of slots can hold a huge amount of weight if you attach them with a few #12 screws solidly into wall studs. The shelf flexing depends on what they are actually made of and how far apart the standards are. I bought laminated MDF and they will sag if there's a fair amount of weight on them. I'd think 3/4" birch would be pretty good. In the past I've used aluminum angle stock or rectangular tubing to reinforce MDF and keep it from sagging.
The actual shelves are 1m long solid birch planks so there is little give in them. There are two rails supporting each one nearish to the ends. But i did only use 2 screws per rail to attach it to the wall. The screws are plenty strong as they go in to a brick wall so i figured that 2 is all i need. I later noticed that if i pull down on the shelf hard enough that causes the rail to bow away from the wall in the middle. This might be where some of the give is coming from.
The weakest link is probably the metal fingers that grab on to the rail. There is a lot of force that wants to bend the finger outwards, they feel pretty strong but once that flat finger bends left or right that then translates the bending moment in to a twisting torque that will want to bend the finger in that direction even more, making for a runaway until it snaps(Similar to how a empty soda can is very strong only until it buckles in one spot). So that's why i'm cautious about putting a lot of weight on it, the actual shelf will give a warning in the form of bending and creeking, these fingers likely go from looking just fine to snapping off in less than a second when overloaded. I did use the cheapest they had at the local hardware store after all.
I probably should have bought 1 extra one to do a destructive test to see at what point it breaks and how, i did buy like 18 of them so a 19th one wouldn't hurt.