The walrus operator has very clear problems once you start to use the assigned variable at the same line it receives a new value. That's not really relevant on practice, since nobody does it, but it moves a language from a philosophy of simple semantics into one where people are ok with undefined behavior or a pile of exceptional cases. This change does smell really bad for people that have seen it before, so those tend to overreact and start complaining that the sky is falling.

Specifically on the case of Python, it used to promise simple semantics a long while ago. This changed at some point, and the semantics were already complex before the walrus operator. With the operator added, semantics becomes a bit more complex, but this is not a main feature of the language anymore, so it's not a big change.

Yet, pattern matching would have a very small semantic footprint, and would solve the same kinds of problems (despite having a very different nature), but was very vocally rejected without any of the deep analysis that went into the walrus operator. It looks like this happened because they would have a very large syntactic footprint, basically completely changing the appearance of the language, but the process left the people that focused on semantics lost, without understanding what was happening.