Jon Favreau, Louis Leterrier, Joe Johnston, Kenneth Branaugh, Shane Black, Peyton Reed, Ryan Coogler, and Scott Derrickson were not untested in any way, and all handled big or very well budgeted films before directing for Marvel.
Jon Favreau - 3 directing credits prior to the MCU all less than $100M budgets (no major franchise experience)
Louis Leterrier - 3 directing credits prior to the MCU all less than $100M budgets (no major franchise experience unless you count the Transporter films)
Joe Johnston - 8 directing credits prior to the MCU all but 2 less than $100M budgets (only major franchise was Jurassic Park 3)
Kenneth Branagh - 11 directing credits prior to the MCU all less than $100M budgets (no major franchise experience)
Shane Black - 1 directing credits prior to the MCU all less than $100M budgets (no major franchise experience)
Peyton Reed - 4 movie directing credits prior to the MCU all less than $100M budgets (no major franchise experience), bunch of TV directing credits
Ryan Coogler - 2 directing credits prior to the MCU all less than $100M budgets (no major franchise experience)
Scott Derrickson - 5 directing credits prior to the MCU all less than $100M budgets (no major franchise experience)
See a pattern, while some of these directors had prior experience NONE had any real major franchise experience with budgets over $100M except 1. Branagh would be the only one I would consider having any clout on this list, but he was a arthouse director for the most part.
Basically all of these guys except Joe Johnston were untested in handling a huge budget movie like in the MCU prior to them joining, and some of these guys only had a handful of directing credits to their name prior to joining.
So yeah I stand by my point, the MCU was built largely on the backs of untested and in many case unknown directors.