[…] For example, \frac{n(n+1)}{2} in latex turns into (n(n + 1)) / 2 in typst. The typst code is incredibly unclear - the first set of brackets with the slash together actually form the fraction operator, so neither end up visible. […]

IMO, it's only unclear if one isn't familiar with the syntax — I think the documentation states the behaviour clearly:

[…] Multiple atoms can be grouped into a single expression using round grouping parentheses. Such parentheses are removed from the output, but you can nest multiple to force them. ^[1]^

This isn't in defense of Typist's syntax, but I challenge you to show the uninitiated your example LaTeX expression, \frac{n(n+1)}{2}, and see if they are able to accurately parse it.