We have recently undertaken the "Great Swift 3 Migration", and even though our migration went well, I am in no way forgiving Apple/Swift-Team for the ridiculous amount of changes, and "death by 2000 paper cuts" aspect. For instance, having the behavior of "force unwrapped optionals" change out from under us meant that our authentication storage mechanism broke unexpectedly.
If you examine each individual change, they are reasonable and most are helpful. But taken as a whole, it was way too much to cram into one release. The core team has admitted as much, because 4.0 is promising to be a much more conservative update.
Also the new
fileprivate scopes are tedious and annoying.