1 - They count pages printed and will simply stop printing when they've determined you've printed enough even if you still have toner in your cartridge.
2 - They will not work with third party toner cartridges.
3 - They won't let you refill your own toner cartridge.
So you are forced in to buying Brother's toner, and and forced to do so before you may actually want/need to.
The starter cartridges even come from the factory with a fill port covered by a removable plug.
The above applies to Brother printers that use TN43x series cartridges. Cheaper models have the reset chip dance.