I think the bottom line is that it's up to the individual instructor, based on the individual student.
Saying, "I think that it's too risky to solo a student at night" is much like syaing, "I think that it's too risky to solo a student if there is more than 5 knots of crosswind". It's just not that simple. I've had students that I would solo in 15 knots of crosswind, and others that I wouldn't solo with more than 5 knots.
Are you leaving yourself open to a lot of liability by allowing a student to solo at night? Without a doubt. However, we do that each and every time we solo a student - even in the daytime. If your number one priority is to negate risk at any cost, then perhaps being a CFI isn't for you.
As I see it, my job as a CFI is to prepare a student for life AFTER primary training. Class C (or B is it's available), night flying, lots of cross country experience - it's all a part of the package.