Are all ascender and descender heights typically equal in well-designed typefaces? Does using identical ascenders and descenders increase readability, or is it not an issue at all?
There are different types of descenders. Consider the difference between a descender like the tail of p, the bottom loop of g, and the tail of j. These are three different designs of descender and these will usually have different lengths.
Similarly with ascenders there will be different lengths between straight ones like b, hooks like f, dots like i and the t which is a cross bar.
However, usually letters with the same type of ascender will have it exactly the same height. For instance, the ascenders on b, h, k and l will usually be the same height.
The height of ascenders above the x-height and the height between the descenders and the baseline will usually be similar, especially for similarly designed ascenders and descenders, but there is never a requirement or expectation that these are exactly the same.
Ascenders and descenders usually are of similar size whereas all the descenders are usually smallers than all the ascenders.
You can appreciate this difference when you are typesetting a text with a fixed line spacing and this difference increases the readibility of the single lines.
Consider also, that most of the type information is stored in the top half of the glyph.
I hope this answers to your question, also this two articles helped me a lot: