The default web interface is very poorly designed and looks uninviting. Sure, there are great alternative interfaces but people will be turned off before they could check them out. Also, it’s usually the first thing you see when someone’s sharing a link.
There not being an official app is also something that will confuse non-tech users.
Well it matters for everything but the rendering engine. Some browsers have extensions, others don’t. Some have poor usability like Chrome and others like Arc work way better.