Qt does not use the standard C++ library, but has its own QString, QVector, QMap, ...
This means you have to make an important design decision: what parts of the application will use QString and which parts will use std::string?
Using std::string in some parts and QString in other parts, means you'll have to convert between QString and std::string on the bounderies.
To avoid that overhead, one could decide to use QString all over your application. But that makes it much harder to use 3rd party libraries that are not based on Qt, e.g. boost.
(Note that the same applies to std::map vs QMap, std::vector vs QVector, and so on)
Deciding which parts use Qt's types and which parts use the STL is a major design decision, with major implications. And only because Qt refuses to use the standard C++ libary.
IMHO, that decision could go either way, depending on the project. So I cannot answer your question which one to avoid.