Like I said, I've seen him several times, at least once a year for the last several years, so I can offer some recent perspective.
First, I believe that your use of the word 'diatribes' is flat-out wrong, as if he is onstage talking negatively about promoters, the band, the crowd, etc. He may go on an occasional verbal tangent but I've never seen him say anything that wasn't positive in engaging with the band or the audience.
As far as the 'vocal numbers' go, that is also a major stretch, as if he is belting out 'My Way' or a string of songs for a substantial portion of the set. He is fond of singing Kenny Rogers's 'The Gambler' and getting the audience involved. On occasion, the band have gamely joined in behind him as he then went behind the kit to lay down a backbeat for a brief instrumental. I remember hearing him once good-naturedly encourage Jaleel Shaw to 'Growl into that mother[...].' Jaleel obliged and uncorked an intense solo.
It's in good fun. He's a showman. He may tapdance. He once said, 'I never thought that I'd live this long...I never thought that I'd be this loose!' At Dizzy's in 2018, he moved his drum solo to my table and started playing on my table mate's magazine (a DownBeat with Tony Bennett on the cover, IIRC). My table mate was a drummer from Chicago who also couldn't believe his eyes.
At one of the last shows I saw, he asked a young guy if he played, handed the kid some sticks and then had the kid play a rhythm on the table, saying, 'You sound good, man!' That surely made the kid's day/week/year. Last fall, I chatted with a Roy fan from Sweden who couldn't believe that he happened to be in the country for a conference at the right time to see Roy. He was glad to see Roy playing so well.
As far as not being 'all there' goes, he's 95. His short-term memory seems affected, but I've seen him talk about his childhood in Boston. He sometimes will go to repeat a song, usually Pat Metheny's 'James', which seems to pretty much be the band's theme song. When that happens, Jaleel will sometimes say they already played it. At one of the concerts last year, Roy just smiled, said, 'Ahhhh...' and struck his snare with a mighty crack. Other times, Jaleel will just transition into another song and Roy will instantly be right there. I've also seen Roy start to repeat 'James' and purposely use it as just a short theme before the next song.
So, nothing major. His playing is so in-the-moment and joyful. I like to sit right in front or next to the drums. Roy can still drive a band like no other or pare his playing down to this minimalist approach that is audacious in its restraint and sometimes unorthodox. His propulsive, unsentimental (yet sensitive) approach to a ballad is still something to behold.
He has his core repertoire and then there'll be a surprise nearly every set, often in the form of a ballad. Roy still leads the band from behind the kit and you'll see him him making eye contact to cue them here and there.
It's been something to see his band develop over the years. They are all top-notch and there is a lot of love on the stand. Last year, Graham Haynes sat in for one set and Nicholas Payton for another. David Kikoski subbed for Martin Bejerano last time and I've seen John Sullivan and Ed Howard sub on bass as well. Roy has responded well to any subs and additions and it is interesting to hear how an additional musician or one sub can change the sound of a regular band. I've also seen Savion Glover tapping like his life depended on it with Roy pushing him.
I guess we all have our own takes on things, but I've never seen Roy and been in the least disappointed. The majority of times have involved that long drive to New York as well, which I wouldn't make for just anyone. Anyone who gets the chance to see him should.