328P has 2k for heap and stack. That's not much room to have storage elements that copy themselves just to add 1 character, not to mention what that could do in time-critical apps.
I see the String Object school play down awareness of both hardware and what the software is doing. They learn to use/reply on wasteful black boxes and don't bother with simpler more direct code. That's okay when you're not pushing much more than halfway to limits. But step much beyond and then your ability to implement solutions begins with learning what you avoided for however long you did including different approaches to solutions that go with all that.
It's easier to match the tools (commands and techniques) to the environment. AVR's tend to be very small to small. What is trivial on a PC is not always going to fit well on an AVR. A bathtub that goes well in a house will be a poor fit on a bicycle regardless of the fact that yes you can fit a bathtub on a bicycle. Oh well let's be reasonable and just fit on a sink instead shall we?