There's a huge gap between knowing enough about rockets to critique their merits and knowing enough to build one entirely yourself. I doubt many car reviewers could build a car from scratch, but they can still make an informed choice between two options by analyzing and comparing various criteria.

As it happens though I do know a fair bit about rockets, so I'm quite happy to discuss any specifics you want.

 

I'd also point out that it's not just random people on reddit levying criticisms at the SLS program, there are some people with serious credentials saying similar things. The Government Accountability Office issued a pretty harsh report on it, and Lori Garver, former Deputy Administrator of NASA, has also had some choice comments and insights.

Perhaps the most notable example is Charlie Bolden, who was the head of NASA under Obama, and as such oversaw the SLS program for about 5 years. In 2014 he (in)famously said:

>“Let’s be very honest, we don’t have a commercially available heavy-lift vehicle. The Falcon 9 Heavy may some day come about. It’s on the drawing board right now. SLS is real.”

At that point in time he clearly had a very high opinion of the SLS program and believed it would come to fruition well before Falcon Heavy. However, after Falcon Heavy ended up launching before SLS (by almost 5 whole years), he had to reevaluate his stance. In 2020 he said:

>“SLS will go away. It could go away during a Biden administration or a next Trump administration, because at some point commercial entities are going to catch up. They are really going to build a heavy lift launch vehicle sort of like SLS that they will be able to fly for a much cheaper price than NASA can do SLS. That’s just the way it works.”

Note that comment was made before the 2020 election results, so when he says "during a Biden administration or a next Trump administration" he's talking about the period from 2021-2024.

And while he doesn't explicitly say who or what rocket, SpaceX and Starship are really the only ones that fit his description.