RE: dx7 and the famous DS821 servos
Heli guys wont touch them because of the sheared shafts. I saw one go on the aileron of a 46 size extra, the output shaft came off the top gear.
I wouldn't put those particular servos in anything that costs more than the servos themselves.
Your milage may vary. Search them on a heli site, there's thread after thread about them shearing or dying.