I still have my DE M&M card which I applied for years ago when I had a German bank account. But as I had no German income they would not give me a credit limit so I have to ensure that there is always a sufficient positive balance and use it as a debit card. They recently allowed me to link it to my Belgian bank account.

I only use it as a backup when Amex is not accepted as it awards 1 mile for every 2 Euros, whereas the SN Amex card awards 1.5 miles per Euro, i.e. 3 times as as many, plus an additional 0.5 miles when purchasing SN tickets - I therefore book my LH and LX flights on the SN site and get issued a 082 ticket and 2 miles per Euro.
And likewise as a SEN I have never had to pay any fees for the SN Amex card, nor for a partner card.

Let's see what the new Belgian M&M card will offer compared to the direct Amex proposal I posted earlier.