Ooh. A kinda loaded, complex, and open-ended question subject to a lot of personal opinion. But don't worry, dear readers, I will not launch into a typical essay-length reply. :-) For that, you can glance at this blog post.
Sykes is an undeniable pioneer in the field. But his studies--and some publications--have not been without criticism. Too, we have to keep in mind that genetic genealogy has been changing as quickly as technology in general. Bryan's last serious book was published in 2011; the last one before that, 2006. I say "serious" because his last, in 2015, was one of those that many academics and geneticists looked down their collective noses at: Bigfoot, Yeti, and the Last Neanderthal: A Geneticist's Search for Modern Apemen.
I'm positively thrilled that his company, Oxford Ancestors, was able to stay in business despite issuing a closure notice last March. We don't need DNA testing companies going out of business, and Oxford Ancestors is one of the oldest. However, in light of advancements in DNA testing in recent years, their offerings are limited.
I, too, own and have enjoyed some of his books, including The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry; Saxons, Vikings, and Celts: The Genetic Roots of Britain and Ireland; and DNA USA: A Genetic Biography of America. But I'll leave it to others to offer additional commentary about limitations and comparisons.