To keep it short: I have over 30 years of experience in IT and programming. Theoretically it is not a big problem to program Arduino board as MIDI controller. But... To program is one, and to test/debug/make it usable and flexible is completely different story.
This board is amazing and worth of every penny. The documentation is very good. Support and communication is also well. It is enormous time saver! If you are student, or billionaire bored to death, go and design/build/program your own solution. If your money counts, and your time is money, then buy this board, and just connect required elements (switches, encoders, pots, displays), use the provided configurator, and have your MIDI controller working in "no time".
I purchased my first Opendeck controller in 2018 and it has been working well with no reliability issues. I recently purchased the latest updated version which also works well and decided to order another one for other projects in the pipeline. The new interface is a triumph of usability and make configuring the controllers very easy indeed. In my opinion, this is the best MIDI controller available and I like the way improvements to the hardware and the brilliant on-line interface continue over time. Keep up the good work!