Brushed motors have been around for more than a hundred years. While there are still sometimes problems with the design, if a company wants to build a durable brushed motor they pretty much can. If problems occur, the source of the problems is usually easy to fix. Aftermarket brushes and switches are usually available, and these are two of the main areas where problems might be caused.
Brushless motors by comparison are much newer technology that uses circuit boards to switch the motor poles. A number of different manufacturers are currently using different designs, with different numbers of motor poles. The circuit boards that switch the motors are usually proprietary. Sometimes the circuit boards are built into the tool switch, and other times they’re separate components, and still other times the switch is built into the circuit board. Current circuit boards in tools are usually potted making removing and replacing a bad component either impossible or annoyingly difficult. Circuit boards are also usually far more expensive to replace than a switch or brushes, and unlike brushed tools with speed control circuitry, you can’t simply bypass the bad circuit board to get a tool functional in a pinch, or due to a bad board design. The brushless motors also rely on sensors connected to the motor or other components, all of which can go bad leaving a tool none functional, or partially crippled. Lastly, the more components a tool has, the more areas where something can go wrong.