What exactly is the big problem here?. Reports from the very few players to hit level 100 are saying that when the items with a level over 100 are crafted they become usable lv99 items.
I don't see a handful of quests that are over level 100 being a problem, by the time you are supposed to be tackling them you are expected to be near to 100 yourself. Unless the enemies are out levelling you by twenty levels I can't see a problem in some difficult late game NG+ content.
If you finished the main game on your original playthrough at around level 35, and play "as designed", then a long, long time later you'll hit Blood and Wine on NG+ at around Level 92. Blood and Wine is supposed to give out around 15 levels, you'll only get 8, so the endgame enemies will be 7 higher than designed. The only reason this is "very few players" is because there won't have been that many (yet) who have gone into NG+ after completing both expansions in the base playthrough - this affects everyone who does. (Or maybe not, see below.)
And I think there's a lot of players who hit level 50 or more in that initial main game playthrough using "legitimate" grinding methods - no exploits, no mods.
So yes, they'd be facing enemies 20 higher than designed during Blood and Wine NG+.
(And although I'm not overlevelled, I've already had enough issues with late-game wolves and kikimores to agree with BMW37007. The random mob-monsters are the biggest problem. A wild dog can kill you with one lunge if your Quen goes down. My approach has been to kick Roach into a gallop and get the hell out of there, but that shouldn't be the way to play the game. But with no XP, why stay and fight them?)
UPDATE: IT'S NOT THAT BAD
The Level Cap seems to be affecting not just Geralt, but the main quest as well, I still haven't finished, but this is what I have so far for recommended levels:
87 - Envoys, Wineboys
88 - The Beast of Toussaint
89 - Blood Run
92 - La Cage Au Fou <I did most of the sidequests and hit Level 100 during this
95 - Where Children Toil <I finished the sidequests during this
95 - Wine is Sacred
96 - The Man from Cintra
100 - Capture the Castle. Roderick level 100, others 99-100. Easier than it was the last time.
100 - Night of the Long Fangs. All Level 100, nothing very challenging.
100 - Beyond Hill and Dale. All Level 100. Pixies were the worst, bossfights were easy.
102 - Tesham Mutna. Detlaff doesn't show, but his pulsating bits in the final segment were Level 102. Fight was similar difficulty to the last time.
102 - Pomp and Strange Circumstance.
If so, then the game is definitely playable if you're on the standard levels or only a little bit overlevelled. The problems and errors were on random encounters. The Main Quest seems to balance correctly.
Of course, if you're a LOT overlevelled, then those random encounters become much more of an issue. And I can't tell if sidequests handle it OK or not, as I wasn't overlevelled enough to trigger it (I wouldn't consider 102 for Tufo unreasonable).
I'd still consider it a bug though, that you can top-out without overlevelling. It wasn't much fun doing all those sidequests for zero XP.