Back at the beginning of the Obama administration NASA had a blue-ribbon committee to investigate what should be done about the over-budget exploration program that was supposed to replace the Shuttle. The conclusion was that NASA should scrap the Shuttle-derived launcher it was working on and develop a "flexible path" program of exploration where rides are bought on commercial vehicles, used to explore whatever the frontier is that is reachable with off-the-shelf commercial technology. This would be paid for by cancelling the rocket and Orion capsule that was being worked on. The legacy aerospace companies that were making bank obviously didn't want the gravy train to end, so they got their bought and paid for congressional reps to literally write in the NASA authorization law a mandate for NASA to design and use their own rocket (the SLS) and the Orion capsule. Congressional staffers even wrote in the exact specifications of this rocket without consulting NASA about whether the specs made any sense at all (they didn't--they were designed to ensure sole source contracts to existing contractors, but with enough changes that lucrative development work would be needed). As such we went from a rocket that was supposed to launch 2-4 times per year, to one which costs 10 times as much and launches once per every 2 years. And one that is even incapable of getting its spacecraft payload to the Moon.
Furthermore, because use of this rocket is mandated by law the entire exploration program around it had to be adjusted to accommodate its inefficiencies. Since the rocket can't get to the moon, a whole space station (Gateway) is built in a totally useless deep space orbit to serve as a destination and staging area for lunar missions. But because of orbital dynamics this prevents many portions of the moon from being accessible to lunar missions, and limits options for direct abort, making lunar missions significantly more dangerous. Also all the effort that could go into making a permanently occupied based on the moon is instead being spent on this useless tollbooth in the middle of nowhere.
My favorite anecdote about the absolute waste of this program is regarding the SLS main engines. The SLS uses 4 modified space shuttle main engines on its first stage. The SLS rocket that is currently prepping for launch in Florida actually has 2 engines on it that were pulled off Shuttle Atlantis before it was put in a museum. These are historic engines that flew many missions in space and arguably ought to be in a museum themselves, or at least cut apart in cross section as teaching tools in engineering colleges. Instead these fully reusable engines, which initially cost about $40M each, were refurbished at a price of $150M(!) each to make them expendable(!!), and are going to be unceremoniously chucked into the ocean a few hundred miles off the coast as part of the SLS launch. That is not a typo: they spent $150M to make a $40M already reusable engine single-use. Actually they spent $600M, over half a billion dollars, to throw away 4 of these engines. Madness.
So the argument goes: this insanity is doing nothing but wasting national treasure and distracting NASA from any meaningful exploration. The SLS is a cancer which has mestasticized to infect the entire human spaceflight program. It needs to be cancelled ASAP, and all the architectures built off it rethought from the ground up if we are to have a national spaceflight program that is in any way meaningful.
(The counter-argument is that cancelling the SLS + Orion programs won't free up money for other exploration projects. This is congressional pork. If you cancel the program, NASA budget will be reduced in the process. Arguably for the taxpayer and the nation this is a good thing, but not for NASA. From the perspective of NASA the SLS + Orion is a free gift of a corrupt congress, and beggars can't be choosers.)