I wouldn't say "never ambiguous" (at least for Scheme):

140 Assignment introduces a subtlety into step 1 of the evaluation rule. As shown in Exercise 3.8, the presence of assignment allows us to write expressions that will produce different values depending on the order in which the subexpressions in a combination are evaluated. Thus, to be precise, we should specify an evaluation order in step 1 (e.g., left to right or right to left). However, this order should always be considered to be an implementation detail, and one should never write programs that depend on some particular order. For instance, a sophisticated compiler might optimize a program by varying the order in which subexpressions are evaluated.

https://sarabander.github.io/sicp/html/3_002e2.xhtml#FOOT140