#3 Getting somewhat exposed by Jenson Button as a wet weather driver
Lewis Hamilton is termed to be one of the best drivers in the rain. It's almost as if when it rains you have to expect a win for the Briton. In the view of the critics, some of those perceptions are a result of recency bias due to his reign during the turbo-hybrid era.
Fans who look beyond the Mercedes years, however, question how Lewis Hamilton fared against Jenson Button (another wet-weather specialist). During his time together with Button at McLaren, Hamilton could not even win a single wet weather race while Button picked up as many as 5 wins in the wet. Button was the stand-out when it came to rain-affected races, not Hamilton.
Lewis Hamilton is surely a great driver in wet weather. How can anyone forget his drives in Silverstone in 2008 or in Turkey in 2020? Terming him the undisputed best, however? Many doubt those credentials.
#2 The lack of contribution in building the Mercedes juggernaut
Mercedes came into existence in 2010. It had Ross Brawn and Michael Schumacher working relentlessly to make it a frontrunner. By the time the 2012 season ended, Schumacher wasn't motivated to continue. For the 2013 season, as Lewis Hamilton stepped in, Mercedes showed significant progress, winning three races and finishing second in the championship.
From the 2014 season onwards, the Mercedes juggernaut started ruling the world of F1. Sure, since 2014, Lewis Hamilton has played a huge role in keeping the Brackley-based outfit at the top. In the eyes of many, however, the transformation of Mercedes from a new team to a frontrunner was already complete by the time Hamilton stepped in.
The success of the Mercedes juggernaut is still considered by many as the legacy of the work done by Michael Schumacher and Ross Brawn, just like they did with Ferrari. That's why in the eyes of many of Hamilton's critics, he just lucked into a great seat and made the most of it.
#1 The losses to Jenson Button, Nico Rosberg, and Max Verstappen
Arguably one of the biggest criticisms faced by Lewis Hamilton is his losses to Max Verstappen in 2021, Nico Rosberg in 2016, and Jenson Button in 2011. These are also probably the biggest deterrents for the claims of him being the greatest to ever do it. While Verstappen beat Hamilton in a similarly competitive Red Bull, Rosberg and Button did that as his teammates.
If you are to be considered the greatest of all time then you cannot lose to other drivers on a similar team. His defeat in similarly paced cars to these drivers is one of the biggest criticisms of his career. It supposedly shows the lack of ruthlessness that Michael Schumacher, Ayrton Senna, and even Fernando Alonso used to have against their teammates.