more than one word....

Servo "a" will not last as long on the same plane as Servo "b".

Now Servo "a" can be from the same manufacture, and can be the same model as Servo "b", or they can be totally different.

One servo can last longer than the wood on a 2 meter glider, but stick it on the elevator of a super 3D machine and it will not last as long.

I've used the same standard Futaba servos on my 2 meter glider for years, but have had to replace the ones in my sport planes.