The best way to avoid criticism is to attack the critic.
So, by saying that Fr. Seraphim has some “well known” critics, were you only referring to Fr. Michael Azkoul and Abp Lazar, or did you have others in mind? If the latter, then please provide links to such criticisms from those whom you believe to be “of reputation”.
From those who are “well known” or “vocal”, who have actually written critical remarks of Fr. Seraphim’s works (mostly regarding the tollhouse teaching), all of these who I am aware of have been closely associated with Holy Transfiguration Monastery in Boston. That includes Abp Lazar and Fr. Michael Azkoul. Reading the book entitled “Letters from Fr. Seraphim” will give a good understanding of the bitter struggle that was occurring in ROCOR between HTM and Fr. Seraphim/Platina in the 1970s, the attempts of HTM to discredit Fr. Seraphim in order to win over the convert section of ROCOR to their “party”, and Fr. Seraphim’s criticism of the HTM group and the direction that they were attempting to take ROCOR. From Fr. Seraphim’s letters you can see that he made some very valid criticisms of the HTM circle, and that he saw them going into schism to create their own church, even a decade or more before such a schism took place. Abp Lazar has since fallen out with HTM, and Fr. Michael and HTM are in a “church” of their own creation which consists of only three bishops in the entire world, all of whom received their formation in the same monastery (HTM). They have communion with no one else because of their “super Orthodoxy” which is little more than a delusion which comes from pride and intellectual vanity.
As Fr. Seraphim made many valid criticisms of the HTM crowd, this crowd may also have made some valid criticisms of his writings, but I find that these criticisms of Fr. Seraphim are largely blown out of proportions and excessive. They do not seem sufficiently sound, objective, or dispassionate. Met Hierotheos of Nafpaktos, an eminent author with a patristic mind who is also a hierarch in Greece, makes mention of the writings of Abp Lazar and Fr. Seraphim in his book “Life After Death”, and I think his views are valuable because of his objectivity and his reputation as a faithful follower of the patristic tradition.
While I do believe Fr. Seraphim is a saint, I do not consider him to be infallible, nor do I think that those who wish to critique his writings are necessarily inspired by the demons. I do think, however, that such critiques should be made as objectively and dispassionately as possible, ideally by hierarchs and theological authors in traditionally Orthodox countries, and that such critiques should have a strong patristic foundation. While Fr. Thomas Hopko may not fit these criteria exactly, I do think his views are pretty objective as he was not a member of ROCOR or involved in the Platina vs. HTM tug-of-war.