From my experience, Luxair is a decent airline, though admittedly I wonder how exactly they are planning to "fit" all this extra capacity at LUX. Only been there twice (once arriving and once departing) so I can't say conclusively that this is always the case... but on both instances the airport was very much packed. And I mean no-free-seats-in-the-lounge packed as well as barely any sitting space at the regular airside seats.
Also, I don't really think the 737 MAX 10 opens up any new markets for Luxair (if we're talking range-wise). They'll probably deploy these to DXB (where the regular 737s already fly to in winter), but I don't think the MAX 10 could even make it to BOS nonstop bar significant weight restrictions. With the A321(X)LR it could've been a different story...