C++ Strings are objects that require memory allocation routines (malloc, free) to be called when they are used. For a long time, there was a bug in the implementation of free that the arduino uses that meant that you could run out of memory, so Strings were a dangerous proposition.
More recent versions of the IDE do not have this defect, but manipulating Strings still requires that you free and allocate memory. Given that arduinos have so little RAM, there is still the possibility that you will run out of space or fragment your free memory in such a way that there isn't a big enough contiguous piece to satisfy a request. At this point, your sketch will likely fail in unexpected ways, which will be hard to debug and when you ask for help on the forums you will be told "Don't use Strings".
There's nothing inherently wrong with C++ Strings if you have the memory for them and suitable error handling for dealing with low memory conditions, but for arduino, where you don't have these things, they are not sensible, particularly if your sketch is expected to run in perpetuity. As an alternative, arrays of char let you see exactly what memory you are using without any 'sleight of hand' stuff happening behind the scenes.