Yeah, SLS is garbage. Literally 1970s technology. Half a century of technological progress was thrown out the window when they designed it.
Side boosters: slightly-upgraded space shuttle SRBs (more fuel, no other advantages)
Main engines: actually, literally salvaged from old space shuttles
Body: modified version of the space shuttle drop tank
Upper stage: borrowed from Delta IV, using the 1960s RL-10 engines
Why is it bad that it's old? It doesn't take advantage of modern production technology, so it's very labor intensive, which makes it both very expensive and makes its reliability highly reliant on the skill of individual workers. What's worse, when these technologies were new, there was a large population of workers with the necessary skills who were eager to become involved in the ambitious, ground-breaking space program. But those skills are now obsolete, and developing them is not attractive to ambitious workers, so you end up with third-rate men doing some sort of experimental archeology project, trying to recreate these obsolete industrial technologies. On top of that, the men who designed these technologies, who also involved themselves in the production, checking and adjusting things to make it work, aren't around anymore. This stuff always barely worked, and needed to be babied along by the people who understood it best, and those people aren't available. In addition, the industrial context this stuff was designed in is gone: things that used to be common, off-the-shelf components and materials now have to be specially made just for this.
This is why it's taking so long, why there have been so many fuck-ups, and why it's probably going to be unreliable if it ever flies at all.
SpaceX kept a continuous team, building from the ground up, using modern technology, and hiring exactly the people they need, generally able to have their pick of the best. Of course they're producing far better results.