A Raspberry Pi Pico as a function generator for arbitrary waveforms

[iMo]

The limitation with an R2R DAC (provided you have got ideal switches and Vref) is mostly with the parameters of the resistors (their precision, TC, noise) and the overall design (the parasitic L and C) when talking high speeds.
In addition you get the differences with the switches (ie. the output characteristics of each Pico output - like their load char, voltage at log1/log0, leakages, switching transients and the noise at the pins coming off the mcu) and the stability of the Vref (its TC, its load characteristics as the Vref's output current may change with set outputs combination).
The monolithic DACs might be much better as the manufacturers usually understand all those nuances well and they strive to make it as best as they can..