We're on our second Sandpiper/Sierra and have had no major issues. The biggest issues we've had was a hydraulic solenoid valve on 366FL that took a bit of time to figure out and the main A/C on our 2015 360PDEK went out, Dometic sent a mobile tech under warranty to fix. Also, on our 360PDEK the water tank fell out but FR quickly took ownership of the issue and sent a factory tech to my dealer with a new tank, mounting hardware and underbelly material.
Other than those issues we've been very pleased with our units and wouldn't hesitate to purchase one.