[inglor_cz]

"why should we trust them to do anything else more complex much better?"

Because the same people previously developed Falcon 9 Block V, the safest and most reliable launcher on record?

[nocoiner]

Isn’t Atlas V also 100% successful?

[inglor_cz]

True, but Falcon 9 Block V adds the partial reuse capability; while we can say that each Atlas V flew reliably once, we can say that many Falcon 9 first stages have flown reliably several times.