Riho's not a tolerable singer
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Sakura is much better than Riho in vocal terms. Her voice has quality and can be very strong at points.
Riho...on the other hand. While her voice may be overpowering,her voice has no real quality in it. Eversince Sakura joined,she has been trying to be a stronger performer (*cough cough* Help me!!),and that is not good for her because her vocals aren't ready for that.
A good singer can be classified as a tolerable singer. A good singer is constantly on point,has quality vocals,and can bring a very overpowering vocals without cracking (Although Sakura does crack every now and then,she has gotten good at controlling her voice.).
A strong singer is a person with a very overpowering voice,but if they were to sing lines in their regular voice,oh hell no. And,who says they are good at a strong voice anyways? They could be constantly cracking.