I listened to both the M21 and the Grimm MU2 at AXPONA 2024 last week. Both are outstanding in many regards, but the Grimm may be a bit better in depth and in resolution. Playing the 4th movement of Beethoven's 9th Symphony through the MU2 was a revelation; I never before heard the chorus so well delineated (in the clarity of individual voices.
That said, I am still tending towards the M21 for my next DAC. My reasons are very down to earth. Bricasti, a US company, has a well-earned reputation for outstanding customer support, whereas Grimm Audio is a new entrant into both the US and into consumer products. In addition, the MU2's reliability is not yet established because it is so new and has so few units in the field. In contrast, the M21 is both a highly flexible and outstandingly performing DAC and one with an outstanding reputation for reliability.
