I agree with the opening poster and have had my colleagues all test in both CC 2014 & 2015 using the following debug:
1: Make a brand new document.
2: Make a text box and type "The quick brown fox jumps over the lazy dog."
3: With the text cursor still active, use the DELETE key (above the \ key on the keyboard) to delete up to the word "brown"
4: Now use the undo function.
In CC 2014, the words " fox jumps over the lazy dog." comes back straight away.
In CC 2015, the letters come back either individually, or in small groups, rather than the whole " fox jumps over the lazy dog."
I've tested this on four Macs all running 10.9.5. I'm with the poster that this is a bug and I'd encourage others to try the test above and see what happens. Filing a bug report in the interim.
Are you sure that you use your normal keyboard and that you have not switched the language of the keyboard? It happens sometimes, e.g. I work here with the Germany keyboard layout (which is called QWERTZ as the first letter line has these letters in that order), and during work it happens that it changes to another layout with QWERTY, e.g. English has this. So the letter Y and Z are exchanged.
I can confirm this anomaly (is it a bug? is it as designed?) in CC-2015 22.214.171.124.
What I observed while testing your "The big brown fox…" example:
Sometimes a single undo will retore a single character, sometimes two characters.
Maybe, if you deleted the single characters at a rapid pace two removed characters will be considered as one single undo?
I recorded this behaviour. You can download the video from my Dropbox account where I tested InDesign CC-2014.2 vs. CC-2015 on Mac OSX 10.10.3:
I also have this problem... Adobe, why does this happen?