Yeah pushing or pulling large, according to salesforce, numbers of records is definitely harder than it should be. On the “push” side you have row lock errors and no way to disable APEX based triggers unless you’ve designed that into the code itself. On the pull side, if you’re trying to extract a large number of records and your soql query times out you’re out of luck. SF is good about creating custom indexes for you through a support case but it takes time. Even then, on the order of millions of records it’s still difficult.

My day job is implementing large SF projects. Multi-million record data migrations arent unusual. Even if the data is clean and mapped the migrations take weeks of planning. We go over every inch of the setup to make sure we have the best chance of getting a clean load on try #1. However, we schedule for 3 trial loads and verification before a “go live load” into actual production. Even after all that it’s still an all nighter with contingency plans and c-suite cell numbers on deck.