Falcon Heavy as complete system is still far more complex. You still need to coordinate all these cores. There is a reason every designer goes to single core as systems grow larger. SpaceX actually had initially planned to do a very large 3 core and then rejected it.

And Falcon Heavy was building on 10 years of Falcon 9 knowlage and an incredibly reliable engine.

Falcon Heavy was launching from a perfectly designed launch pad that didn't throw up huge chunks of debris everywhere.

Its incredibly that people are way to fast to jump to doomsday conclusion from one test.