The basic answer is that Bembo is universally accepted as the artist now, I believe. You can see, if you have Kaplan II, that the same artist who drew the illustrations for the Lancelot du Lac of 1445 also painted the Cary Yale and the Visconti Sforza cards. Kaplan shows other paintings attributed to the same artist - I can't remember if Bembo is positively identified as the artist here, but they are obviously the same artist and I think records of commissions might have made it certain that it is Bonfacio Bembo.
The long story is that in the early 19th century, Count Cicognara published the Visconti di Modrone (Cary Yale) cards for the first time, and attributed them to Marziano da Tortona. He mentioned (or published some?) of the Visconti Sforza, and attributed them to an ancestor Cicognara. The document he based this theory on turns out to be a hoax, whether done by him or perpetrated on him is unknown.
Bembo floated up in the early 20th century, but so did Zavattari - Zavattari's (or the workshop of) works are attributed, but no work of Bembo's had been positively identified. By the time of Moakley (1956/1966) and Robert Klein (art historian, article of 1967), Bembo was the favorite. In the early 1980s, Giuliana Algeri (art historian) resurrected the Zavattari thesis, which accounts for some of Dummett's remarks. But Bembo remained the general consensus. Kaplan's 1986 Encyclopedia showed all these works of the same artist, and by 1999 Sandrina Bandera, curator of the Brera Museum in Milan, brought the Lancelot du Lac argument and other works - now positively identified as Bembo, I believe - to the forefront.
In 2005 (IIRC) Dummett suggested that Bonifacio Bembo painted 14 of the trumps (and the rest of the deck), and his brother painted the other six, probably in the early 60s.
That's the state of the question now I believe. But Bembo is universally taken as the artist now.