My 5 year old Howdens kitchen is more solid than anything I've seen in
the sheds


My 25 yo schriber units are still solid.
They aren't flat pack and arrived assembled.

I suspect that if you glue the flat pack stuff it will be just as solid.

My Howdens were not flat-pack

I've never had a cabinet fail, flat pack or not. By the time you've
screwed them to the wall, each other, and the worktop, they are really
over-engineered. Hinges, drawer bases, and drawer runners: now that's
another different thing entirely!