I very, very strongly disagree with this. On web, Flutter is flatly awful (and there’s no reason to expect it to get much better—it’s chosen a paradigm that makes it impossible to do a good job on the web). On desktop, well, this article describes various limitations and problems.

Flutter is unquestionably designed mobile-first, with everything else a distant second. Flutter may be the best option for developing cross-platform mobile apps right now (I don’t know), but if you care about web or desktop, it’s not a particularly good choice. Things based on web tech are very likely to be better.