Materials vary. Mills Pride is all melamine coated particle board on the sides and bottoms with solid doors. At least the melamine (wood grain) has very good waterproofing, but over time, the particleboard WILL weaken, especially where hinges are connected. The less expensive cabinets have no face frames but are "Euro styled" such that the doors are the cabinet front. Once you get into real face frames, you are talking about a real cabinet. Most are solid hardwood face frames and hardwood plywood sides, toe kicks and supports, with possible melamine particleboard bottoms and shelves. Materials vary widely and you get what you pay for.
You know the dance. If you use particleboard in the bath your cabinetry is as disposable as the razor you shave with. You know Henry, I can't figure out why anyone would use this in a sink base. The cost savings of particleboard over hardwood plywood is probably no more than $100/average cabinet. When you figure the life of a particleboard cabinet might be 5-6 years and a plywood carcass on the order of 18-20 years, it just doesn't make sense. When a cabinet goes bad, you often lose the countertop and have all the plumbing and sink issues to deal with. It adds up to a lot more work than using higher quality to start with.