We also have the 4x4 set, but the plastic tubs (same blue, red and clear) can have sharp edges if over time, wear and tear etc get the better of them.

We only have the tubs on the bottom shelf though as I could foresee one of the kids using them as leverage ( if on a higher shelf) which could see the entire unit fall over if not secured to the wall. We rent, we can't attach our unit unfortunately.

But big thumbs up! It's a great unit for kids toys