This article[1] describes him as "ruthless" and unnecessarily critical, and I'd assume that's most people's issue with him. It has recently resulted in him not giving young drivers much of a chance to "find their footing" before demoting them (e.g. Kvyat, Gasly, Albon).
[1]: https://www.planetf1.com/features/formula-1-villain-helmut-m...