I've been told that the use of ellipsis in "$S = x_1 + x_2 + x_3 + x_4 + \dots$" is ambiguous and meaningless. Is it?
Stating ambiguity of this summation is bad faith. The pattern is obvious and the ellipsis clearly indicates an unlimited sequence of terms.
I would be more critical towards a sum like
$$1+2+4+\cdots\ ?$$
In common practice, if the first few terms/indexes (as little as $3$) follow an arithmetic progression, it can be considered implied.