Robert W. Baloh and Robert E. Bartholomew criticizes both studies and much more in their book Havana Syndrome: Mass Psychogenic Illness and the Real Story Behind the. Please refer to that book for details.

Stories about this mysterious illness first appeared in the press in early 2017. At that point, many of the symptoms reported were auditory. Diplomats reported suffering from tinnitus, loss of hearing, and of hearing noises. Some of the diplomats recorded the sound they heard and in this AP segment you can hear the sound being played:

https://www.youtube.com/watch?v=rgbnZG85IRo

Note that the segment is rife with speculation. The reporter speculates about there "possibly" being additional inaudible frequencies too low or too high for recording devices to pickup "when these attacks occur".

Two years later, entomologists perfectly matched the sound the diplomats heard to the sound of Cuban crickets:

https://www.theguardian.com/world/2019/jan/06/sonic-attack-o...
https://orthsoc.org/sina/492a.htm

Thus, the only tangible evidence of these "sonic attacks" turned out to be false. That should have been the end of the story, but, alas, it was not.

Regarding the 2018 and 2019 studies, while the author list is organized slightly differently, both have the same authors. So if the 2018 study is "shoddy" it reflects poorly on the 2019 study too. Baloh and Bartholomew writes about the 2019 study:

"On July 23, 2019, JAMA published a second study of US Embassy diplomats who fell ill under mysterious circumstances while working in Cuba. A team of scientists including some who were involved in the original 2018 JAMA study, conducted sophisticated MRI scans on the brains of 40 diplomats who reported symptoms. The real mystery of this study was how, after receiving a torrent of criticism of their earlier publication, the editors could have approved such a flawed second study. Once again, their findings generated alarming international headlines as researchers found that their white matter volume had shrunk by about 5 percent, while their auditory networks exhibited a 15 percent reduction in function. The researchers wrote that “imaging revealed significant differences in whole brain white matter volume, regional gray and white matter volumes, cerebellar tissue microstructural integrity, and functional connectivity in the auditory and visuospatial subnetworks” when compared to a group of healthy controls [18, p 336]. To the layperson and journalists who were not neurologists or experts on brain function, this highly technical study could easily be seen as confirmation that something sinister had transpired in Cuba. Indeed, publication of the study prompted a flurry of media reports which gave credence to the acoustical attack claims. For instance, Britain’s Sun newspaper reported that the mysterious ‘sonic attacks’ had left US embassy staff in Cuba “with shrunken brains” [19]. The New York Post proclaimed: “Cuba ‘Sonic Attacks’ Changed US Diplomats’ Brains, Study Finds” [20]. New Scientist heralded: “Brain Scans Hint the Mysterious ‘Sonic Attack’ in Cuba was Real” [21]. Reuters quoted the study’s lead author, Dr. Ragini Verma, as describing differences in the brains of the two groups as “jaw-dropping” [22]. However, this was not borne out by the data as differences between the two groups were relatively minor. This finding was buried in the limitations section where the researchers were forced to acknowledge that “…it cannot be determined whether the differences…are due to individual differences between patients or differences in level and degree of exposure to an uncharacterized directional phenomenon.” In other words, the differences could be potentially explained by individual variation. Curiously, this limitation was never mentioned in the abstract, which gave the impression that affected embassy staff had brain abnormalities that could not be accounted for. At the very least, the wording made it appear as if there was more of a mystery than there was. The study was rife with methodological flaws and claims that were not supported by the evidence. While they found brain anomalies, their significance was unclear. Brain changes do not equate to brain damage, and 12 of those affected had pre-existing histories of concussion, compared to none of the healthy controls. This alone could account for the differences between the two groups."

What we have are two shoddy studies, recordings of Cuban crickets, and a number of self-reported vague symptoms. This evidence does not point to a secret, undetectable microwave weapon developed by the Russians and deployed against Western diplomats...