Having just spend the better part of 3 months porting a lot of legacy projects, you're absolutely right, but I am not sure where MS said this would be smooth. There are some processes that are standard enough but certainly there is a lot of hands on work involved, and actually for good reason, there are performance and security issues that need to be addressed (In my brief, and hopefully final experience).
That said, porting from event .net versions (stable LTS versions) such as 6 -> 8 has so far been flawless, I think over the last year, we ran into maybe some minor semantic issues.
It's not perfect, but I have to defend MS unfortunately, they are taking things in the right direction while being tied to a LOT of legacy enterprise clients and systems.