I posted in the support thread that motivated this one, but there’s at least one instance where copying and pasting part of an answer looks good inside the answer box, but the new preview showed “-”.
I think at least in part this is because the trig functions are being pasted with extra spaces before the argument. I copied and pasted from the correct answer I made visible in order to help with debugging, but is this actually a feature and I should tell students that if they copy-paste answers in from “somewhere” they’ll likely be marked wrong?? 
Anyway, this is the kind of thing I was trying to catch when I asked for a preview. Another example would be that earlier on in the course, a student had a double exponent when they didn’t mean to but the MathQuill renderer looked fine because of tall parentheses. Duane fixed that already though.
One thing I was hoping for was some additional parentheses help, and it doesn’t seem the new preview is providing that. I saw at least one student do the quotient rule on something kind of complicated and they didn’t have parentheses in the correct place in part of the numerator, but it wasn’t super obvious - all sets of parentheses were the same heights and whatnot. So maybe there’s more that could be done with that to make a preview more useful?
I agree with everyone else here that authors opting in for only some questions is fine. There are plenty of questions with answer blanks where MathQuill is enough and an additional preview would only be an annoyance.
Oh, and I did notice that if the width of the activity is small, as it can be within the editor/previewer on doenet and also within PreTeXt books, then the new preview appears above the answer blank instead of to the side.