Muun isn't a Lightning wallet because Muun does on-chain swap which results in some people getting very high fees. I have personally never experienced this, but I've seen countless discussion around the fees being as much as on-chain fees when you're unlucky enough to be the person that has to pay for the channel being opened or closed. My 2 kids have used Muun for 2 years without issue or high fees.
Discussion
In WoS your fees can go as high as 100% of your total holdings (if they decide to rug you). In theory this can't happen in Muun (assuming they use proper submarine swaps, which I don't know if it's the case because they are not very transparent about this process).