First, I always appreciate the effort for creating open systems:

and it's entirely open source — bar its off-the-shelf print heads and ink cartridges.

...but the cartridge is usually the worst offender of commercial implementations for a number of reasons.

...leading companies including Brother, Epson, and Hewlett-Packard to implement a range of restrictions in hardware and firmware in an effort to lock printers down to their specific first-party cartridges.

The Open Printer, its creators claim, won't do that — although it's based around off-the-shelf Hewlett-Packard color and black ink cartridges with built-in print heads, the tiny microfluidic nozzles of a high-resolution inkjet being a little beyond the realm of do-it-yourself hardware. These cartridges, which can be third-party compatibles or refilled originals, are installed in a cartridge board driven by an STMicroelectronics STM32 microcontroller — which is, in turn, connected to a Raspberry Pi Zero W single-board computer acting as the central brain of the system.

So they've built their own driver for the cartridge which is good as it would prevent the vendor from denying the use of third party or expired first party cartridges from operating. There's still the expense of acquiring the first party cartridges, and the questionable quality of third party/refilled to contend with.