They must have some consistency issues, the locks were the only thing on mine that didn't break!!
I would agree that the plastic is very hard to break in a fall but I found that mine stretched and distorted over the thousands of little jolts so the seals were virtually useless after a few 100 miles of corrugated track.
Mine were pretty full, as I imagine yours will be, so opening from the side can be a real pain if you just need something quickly, and opening them for customs searches was hilarious - 'Open this for me', 'I'd really rather not' - now suspicious customs officer now makes me open them at which point my folded funnel insists on unfolding at speed and creates a mini luggage bomb, neatly spreading tools, clutch plates, spare cables and dirty socks. They only ever made me open one.