I think it was a mistake to map neighbourhoods in Brussels as boundary relations, because the definition of them is always quite blurry.

There is one official definition of those perimeters, used for statistical purposes.
https://monitoringdesquartiers.brussels/Indicator/IndicatorPage/2336?Year=2019&GeoEntity=2&SubTheme=28

If you ask me, I’d recommend that we only keep place=neighbourhood nodes on various places, without tracing fixed borders, because that will always be a controversial choice.