Ive always liked Rachel's smooth service action but you're right it lacks a bit of power and whenever Ive seen her play in the past she just doesn't get enough of them in.
But that can be said for her whole game, she really lacked a killer shot to reach the top 15-20. And though she certainly has the fitness and variety that would be the envy of many players.... her groundies, which as a baseliner should be the bedrock of her game, can be very flakey.
So when she was 18, she was touted as a future star - but after that her groundies never kicked on (too flicky to be reliable and a lack of power to be a weapon) and I think got a reputation around the traps that she was suspect mentally and that if you stayed with Rachel she would crack.
There's more to life than just being happy.