While I'm not a specialist with connection times, all I can say is that Amsterdam is so much SMALLER and so much EASIER to transit compared to Frankfurt or Paris (or even DTW and MSP for that matter). Based on my personal experience, 50 minutes should be more than enough time to change planes unless your Prague-Amsterdam flight is delayed.

On the connections I take via Amsterdam, I am often stuck with 4 to 5 hour layovers. I personally would welcome a schedule with a 50 minute layover in Amsterdam and would take it in a heartbeat if it were offered on the routes I use. I would simply consider three factors: (1) whether you could potentially afford to experience a forced overnight if you miss your connection; similarly whether there are later flights to the U.S. the same day so NW/KL could rebook you if you miss the connection; (2) if you are concerned about making the connection because of the walk between flights, you might consider calling reservations and requesting a handicapped cart for the connection if that is offered; (3) With a 50 minute layover, I would assume you would be transferring directly between flights, no time for airline clubs, duty-free shopping or other sightseeing in the airport.

Personally the walks I have experienced between flights in AMS are far less distance than changing planes in DTW or MSP