Heck I used to read that way back in the day they only used plastic gear servos in large gassers.
But don't do it!
I almost lost my Topflite Gaint P40 last weekend. Im not sure what I was thinking at the time of building this one but I had Futaba S3010 plastic gear servos on the Ali
While I installed a new Iceman 3axs Gyro on it and was on my 7 flight of the day adjusting the gyro I almost lost my bird. Even with 2 6.6life 2500 and 2 switches the survo put my plan in black out for 2 sec.
After I got it down I found out the left one completely stripped and the other side was striped a few teeth and was about to let go
HS645 are great low dollar gaint war bird servos