Stock cabinetry comes in fixed widths, usually three-inch increments. Your kitchen does not. That mismatch is why so many otherwise nice kitchens have a filler strip where a drawer should be, a pantry that stops a foot short of the ceiling, and an island that leaves one walkway two inches tighter than it should be.
Custom cabinetry solves that by starting from the room instead of the catalog. The run reaches the wall. The pantry uses the full height. The appliance garage fits the appliance you actually own rather than the average of everyone's. None of that is luxury, it's just arithmetic done in the right order.
The second difference matters more on this coast: what the boxes are made of. Southeast Texas spends months at high relative humidity, and moisture is patient. A particleboard carcass swells at the edges and never comes back. Thin veneer delaminates at the seams. Neither failure shows up in year one, which is exactly why they're easy to sell.
A plywood carcass with a properly sealed edge handles that environment for decades. So do finishes chosen for humidity rather than for how they look on a showroom floor in a climate-controlled building. If you're replacing cabinets because the last set swelled, replacing them with the same construction is an expensive way to schedule the same problem again.
The practical compromise most homeowners land on: custom where the room is irregular or heavily used, which is usually the kitchen run and the primary vanity, and simpler construction where it isn't. Measure first, decide second, and don't let a catalog's increments dictate the shape of a room you're going to stand in every day.