The aperture of the Color Muse is about 1 cm and the Nix is about 1.4 cm.
The Color Muse and the Nix both use a sensor developed in Germany by the company MazeT. The sensor is small and cheap.
The tristimulus sensor leads to one limitation for both devices. Even if the sensor were an absolutely perfect match to the tristimulus functions, it can only see the sample under the lighting that the device has. If a spectro is used, you can predict D50 or illuminant A or F10, or whatever. With a tristimulus based sensor, you have to resort to the Bradford transform, so there is error introduced.
Both products use white LEDs as the light source, so that is the illuminant that you are measuring under. But, the white LED used in both is different from the white LEDs in a flashlight. They both use multi-phosphor LEDs that don't have a deadspot in the spectrum just below 500 nm. In other words, they see color better. Both are perhaps similar to D50, although the Color Muse is a bit closer.
The biggest issue that I saw when I evaluated them was that the lighting in not 45/0 in either one. In the Color Muse, they made an attempt at 45/0, but in reality, it is closer to a spherical instrument. The Nix is actually closer to 45/0, but they have a large amount of light coming in at close to perpendicular. What this means is that it is not possible to calibrate the instrument to match a standard instrument on samples with different gloss. It may measure a glossy surface well, but won't do so well with a matte surface. Or the other way around.
Another issue is that neither of the devices have any UV component to the lighting. That means that you have basically an M2 instrument.
I measured a collection of white / gray / black objects with various amounts of gloss and OBAs. I found errors of up to 4.0 DE76 for the Nix. Errors were fairly small when you measure a Pantone book, small enough that I suspect that they calibrated on a glossy stock with a moderately high OBA content. The Color Muse had errors of up to 7.0 DE76 on the same samples.
Is this acceptable for printing? Perhaps for monitoring color during the run, but certainly not for checking at makeready.