The Reader's Digest Version:
It appears that Align's DS520 servo will not work reliably with the Futaba GY520 gyro.
The detailed version:
When powered on, the GY520 pulses the rudder servo during initialization. During a recent build, I'd hoped to use the new GY520 gyro with the servos included in the Align T-450 Pro kit. During bench setup of the electronics, at first I thought I had a bad rudder servo, but it worked when connected directly to the receiver.
I subsequently discovered that the servo would operate with the GY520 if the servo was unplugged and plugged in again after gyro initialization. Thinking perhaps the GY520 was the issue, I tested with a number of other servos including S9254, HDS877 and the Align DS420 that is bundled with the T250 - all worked flawlessly every time.
I finished the build using the GP750 gyro that came with the kit and resolved to revisit the GY520 gyro later. The next day, I bench tested the GY520 / DS520 combo and it worked several times in succession - in fact I couldn't get it to fail.
So I tore the heli apart to replace the gyro - and of course it stopped working again. After repeating successful tests with other servos (all worked every time), I thought perhaps I had a flaky DS520 servo so picked up a replacement at my LHS.
The replacement exhibits the exact same symptom - it's worked only once in ten tries when connected to the GY520. If the servo doesn't move during gyro initialization, it doesn't work at all unless unplugged and plugged in again.