I use cubes and jerrycans as fermenters all the time, brewed many award winning beers with them and they have 40mm to 60mm openings...
You make a lot of claims about how things are unsuitable etc or your products are "better" but there seems to be little to no actual proven basis for those claims
When we distributed the Fermentasaurus it was a common customer complaint that the opening was not big enough which is why we designed the FermZilla with a much larger opening at the top. Certainly it's possible to ferment with very small openings. Some customers ferment in juice bottles and also get good results. With that said it's not preferable but the vast number of customers that we talk to. Having a fermenter that you can get your arm inside and wipe off the krausen is something that many customers have specially said they want. We sell these Cubes for $9.95 and if they really made such good fermenters I am sure we would see a lot of people choosing this very inexpensive option. I think if we sold this as our recommended fermenter we would have a lot of customer complaints.