The title says it all: what do various sources say regarding the authenticity of the Meggid Mesharim; a book supposedly containing angelic revelations to R. Joseph Karo. Did R. Karo really write it?
I am including scholarly sources regarding this historical question of authorship.
Firstly, another scholarly resource that confirms this is R. J. Ẕevi Werblowsky in The Character of R. Joseph Qaro's "Maggid" /לדמותו של המגיד של ר' יוסף קארו He states (fn 1, my rough translation) that all of the suspicions and claims that have been heard are null and void. (He also notes another book written on this topic, read the review here if you dare.) His opinion is significant, as he is the author of Joseph Karo : lawyer and mystic, where he tackles this issue at length. A short excerpt from the section dealing with this issue is available here, where various proofs are presented in favor of M.M.'s authenticity. Werblowsky's book is reviewed here by Joseph Dan.
Secondly, I'm not sure if this is exactly what you are looking for but in Shadal's Vikuach at Chochmat Hakkabalah he devotes a chapter to the Maggid Meisharim. Translations and notes are found here on Parshablog (see part iv and previous 8 posts). In part i he begins by quoting the Sefer Emunas Chachamim, Ch. 27 (which is also quoted by Chida in Shem Hagedolim), which brings some of the proofs suggested by Marvin J. Heller in Yishai's answer. Shadal considers the argument based on Sefer HaPardes (a sefer written by R' Moshe Kordovero in the lifetime of Rav Yosef Karo) to be a straightforward proof that the M.M. could not have been a forgery. Josh Waxman (translator there) provides Mevaqesh's counter-argument that perhaps "someone took that genuine quote and included it in a forged Maggid Mesharim to bolster its authenticity...". However, see the full quote from Emunas Chachamim, as this bolsters the argument, by noting the reputation of the Sefer, as well as the likelihood that it was read by Rav Karo:
וא"כ איך אפשר שדברים כאלה המדברים בסתרי התורה יכתוב הר"ם קורדואירו בשם רבו מהרי"ק בחייו ובעירו של מהרי"ק ויעלם על ספר ויפרסם אותם בפני רבו וישלחם להרמ"ע, ובכמה מקומות אחרים אם לא היה האמת שהרב קארו כתבם ופרסמם יתעקש המתעקש ויאמר מה שירצה שזו היא בעיני תשובה ניצחת שאין להשיב עליה והראיתי דבר אלה לאחד מן המתפלספים ואמר לי שצריך לומר שהרב קארו בדה דברי שקר כדי לגדל מעלתו, תאלמנה שפתי שקר אתפרכן אתחרשן הדוברות על צדיק עת
Shadal there assumes that if we would not consider Rav Yosef Karo to be a "completely wicked individual, like Yeravam and his colleagues" and as a result, it is impossible to believe that "he invented these words from his heart". (Note that later in the Vikuach, there are some passages that are pointed out that would be hard to accept as having been said by an angel, or accepted by Rav Karo. See here.)
The above, however, is simply a review of the introduction of this section. Shadal's main thesis is that Maggid Meisharim was never intended to be literally taken as a discussion between Rav Karo and an angel, but rather it was just him going through his thoughts in an imaginary dialogue. His proofs for this are as follows:
Hinted to in this statement:
(Venice printing, page 43c, Zalkwa printing page 50a {?} ): "Is it not so, that even though at times I explain some explanation on a verse, which is not so, am I not saying according to that which in your will."
various statements noted in parts iv - viii of parshablog links
The statement of the Maggid (Venice, 47a, Zalkwa, 51b):
"It is I who speaks with you, your soul {neshama}, not the nefesh nor the ruach, but rather the neshama herself. And is it not so, if prophesy has ceased from Israel, from you it has not ceased, for each time I come to you to direct you in which way to go."
hinted to in the introduction of Rav Shlomo Alkebetz which was written in the beginning of the Lublin printing
These proofs are hammered out in the parshablog posts above, I would suggest that anyone interested read them directly, as well as various other related parts of Shadal's Vikuach.
This is apparently also the view of the Chazon Ish (here, Section 6, h/t to DoubleAA), and R' Jose Faur (as per Mevaqesh).
Well, you wanted scholarly sources, so here is one in "Seventeenth Century Hebrew Books Volume 2" by Marvin J. Heller 2011 p. 665:
Caro's authorship of Maggid Mesharim has been disputed. A number of writers, particularly those who did not wish to attribute a mystical work to a leading halakhic authority, questioned Caro's authorship, an attribution that today is unquestioned. Contemporary evidence comes from R. Solomon Alkabez, who wrote that he and others heard the maggid speak on Shavu'ot night; R. Hayyim Vital refers to the maggid in Sefer ha-Gilgulim; and R. Moses Cordovero has a quote in Pardes Rimonim (1591) found in Maggid Mesharim. Gordon points out that Caro's children never expressed any doubt as to his authorship. R. Shalom Bailea (1680-1743) provided a detailed defense of Caro's authorship, and more recent scholars found no reason to question his authorship. Werblowsky writes that "the manuscript and textual evidence in favor of the genuineness of the M.M. is greatly strengthened by the internal evidence of the book."
SJL Rapoport was of the opinion that R. Alkabetz was the author of MM and not R. Karo (Iggrot Shir, no. 37). His main contention is that R. Karo appears to never express regard for the book (nor does his own son list it among books authored by his father). No kabbalistic lessons communicated to him are ever cited, advanced or relied upon.
