Putting the following in the paint event of a widget:

p.drawText(rect(), Qt::AlignCenter, QStringLiteral("a☢️a"));

will give you centered text with the coretext backend but not with Freetype. Most likely it is because Freetype does not support the default SBIX font so we end up in some weird fallback path, though this has not been verified yet. At least there are messages such as this on the console:

FT_New_Face failed for face index 0 : 90
QFontEngineFT: Failed to create FreeType font engine