I am one of them 
Answers are in the order of your questions.
Biggest challenge was to get accustomed to the product. While many things are easily understood, some concepts like content views and content libraries are something entirely new. There is no documentation available for former Spacewalker’s trying to make the transition.
I would have loved to use a migration documentation, or a how-to, in order to make the transition. While a few docs are available by searching online, they all fall short of fully explaining a simple migration process. To be more exact, they basically stop at the moment you have your repos synced and all added to one single CV. Which you will pretty quickly find out is a stupid idea. But that’s way too late.
Biggest obstacle, I would say is the product stability. I know this sounds a bit harsch, and I ensure you its not meant like this, but I had to reinstall Foreman/Katello 3 times before I got it to work. Please keep in mind, this is a migration, not someone installing a new linux management solution in-house. So I mirrored all my repos and hit some roadblocks there, like DB corruption. Then setting up remote smart proxies is a nightmare, as the syncing fails for some repos (being looked at right now). Tons of 500 errors when downloading packages, hanging tasks … you get the idea. Eventually after the third setup I knew what to avoid (more or less) and could move forward.
Oh yes, I would have loved to know beforehand how to deal with CV’s, to NOT subscribe your smart proxy to the Library (why does it even allow you to do that?) and that there is practically no automation of any kind to keep up with updates. For the latter, without errata’s (whose import don’t work for CentOS 7 at the moment) there is no email about new packages, no info about any kind of changes. You need to manually click through all your content views, aggregated views to update and publish them. Clean out the old versions. This is a tedious, cumbersome and very time-consuming task. I would say, folks coming from Spacewalk need to know, that you are constantly required to manually perform actions in Foreman/Katello, otherwise your linux systems will not be up to date. I still haven’t gotten my head around that and its one of the biggest obstacles to overcome.
The most help I found was in these forums. Folks in here are friendly, helpful and mostly quite immediate in responding. The documentation of Foreman/Katello is well written but doesn’t really explain hands-on procedures. Its more like an explanation of each setting with a few how-tos, but the overall concept is hard to understand from it.
Last but not least. The product is great, migrating is the way to go. Spacewalk is dead and there is no alternative to Foreman/Katello IMHO. I appreciate the help of everyone in here, form the development side and the community.
My 2 cents.