My first thought was their exceissve weight, second was Ikea cabinets are seemingly designed solely around being able to be packed flat in as small a box as possible, and be quickly assembled at home by an illiterate person with little to no mechanical aptitude, third is the particle board/formaldehyde factor.
I've modified ikea cabinets, they have their uses, but the smell when cutting that particle board is chemical nasty and lingers.
Also, when the particle board gets wet it swells and soon looks like shit stained cellulite.
But on the formaldehyde note, i made extensive use of plywood in my vans cabinetry and bed platform and underbed storage. It is sealed with polyurethane. I've been exposed to nastier stuff. Lots of ventilation helps mitigate those concerns.
See if your area sells baltic birch plywood. It has more plies and thinner glue lines and is lighterweight and more dimensionally stable than home store plywoods. Baltic birch is the name of both Finnish birch and Russian birch plywood. The Finnish birch is more desirable with less voids allowed but as expected, more expensive. It is often sold in 5x5' sheets instead of the traditional 4x8. Usually a very pretty outer veneer which takes a stain nicely too.
Seems most people overbuild bed platforms and van cabinetry with big 2x4 frames and plywood covering that. If I need a sub structure it is 1/2" x2" fir cleats I ran through the tablesaw, or well chosen knotless 1x4s i rip in half on the tablesaw.
Depends on what is available and how much strength is needed.
For making cabinets fit the curves of a van wall, Make cardboard templates, stapling or taping pieces together and cutting it with tin snips, then transfer that template to the plywood.
The Kreg pocket screw kit can make really nice strong joints without a substructure. Glue and pocketscrews is even better.
