My memories of monoblocs suggest that the problem is less that the chair can't take the strain of being stood on, and more that it can't take the strain of being climbed onto. They're not rigid bodies; getting on to one to stand up will shake the seat around like crazy.

Most chairs are much more physically stable.

I had many steel chairs break at the welds. Or have the legs bent. For example, if you balance on the back legs, both the steel and plastic chair may collapse, but the plastic chair will keep its shape afterwards, while the steel chair may stay bent or break.

I'm a relatively tall guy and am wary sitting on these, standing on them would be certain failure. They are essentially cheap trash that will spend ~12 months looking like a chair and 500 years looking like a broken chair in a landfill.