They didn't get it perfect, but, compared to what it was like to deal with Unicode in Python 2, I'd say they absolutely succeeded at improving the experience of working with Unicode text.
In my uninformed opinion they should have simply added the u"..." and b"..." formats and forbid mixing them or using a u with a b operation and viceversa.
python 3 could have simply changed the default from b to u for untagged strings.
The problem was that this made a muddle of all the string handling functions, which needed to be prepared to handle both types. Including ones you write yourself. And, Python being a dynamic language, getting this right was fiddly and error prone. "Forbid mixing the two" was a workable, if ergonomically annoying, in statically typed languages that predated and then subsequently had to adopt Unicode. For Python, though, it was an enormous minefield of runtime errors.
$ python2
>>> "abc" + b"123"
'abc123'
>>> "abc" + u"123"
u'abc123'
$ python3
>>> "abc" + b"123"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "bytes") to str
>>> "abc" + u"123"
'abc123'
>>> type(u"123")
<class 'str'>