If I had to guess, I'ld say it is about them being an on-chain wallet but they say they support LN. Muun only does submarine swaps for providing LN support.

Muun's claims of being a Lightning wallet are dishonest: costs $0.5 to send $5 #94389 https://twitter.com/peterktodd/status/1592532859616915456 [Nitter]

Another allegation possibly is that the app in the Play store is not "reproducible" from the source code, per Wallet Scrutiny's methodology which requires a deterministic build to result in the exact binary that is released on Play store.

Muun even acknowledged this:

https://twitter.com/MuunWallet/status/1561730856229675010 [Nitter]

But many still blame Wallet Scrutiny's methodology that does not (and probably should not) account for things like the inert issue that occurs with the build process used by many apps, as well as how Wallet Scrutiny is not current on their reveiews, so even apps that may have fixed whatever are still showing as "not reproducible" because Wallet Scrutiny has far too few resources for such an ambitious endeavor.