I have not migrated yet! Preparing for this for a client. The site is polyclinic.com. When I inherited the D6 version, the theme was a test case of everything wrong - a template built site in which the developer used mysql_query() in the node template to get node elements. I convinced them to spend a lot of money for a proper theme upgrade so upgrades would be easy after that, and then we get D8 - argh!
Anyway, It's a site of hospitals and clinics and tons of node references between clinics, the doctors at each one, and their specialties. I assume node references migrate well! My module analysis shows that very few really required contrib modules are missing. Yay! I have a few questions as I prepare:
1. We used a ton of Views. Most pages are collections of View blocks. What kinds of fields or formats will not migrate (aside from php fields, which I will need)?
2. Will Metatags migrate fully? Webform?
3. What system does Backdrop use to build a View Google map of geocoded locations in nodes?
4. Modules I think I really need that I have not found: Cookie Crumbs, Login error, Imagecache Canvas Actions, Local Menu. Are there other ways to do these things? If not, these are probably easy to convert, but thought I'd ask if there are existing options of a different name.
5. We use the theme Adaptive. No mods to it's structure, but our long CSS files mostly are theming views. Are the ids and classes from Views the same once we get past the node and page changes documented for converting themes?
6. Lastly, how do I go about estimating hours for this conversion? If you're not inclined to suggest how many hours it will take, after I estimate the parts I can guess, what percent would you add for everything else that inevitably shows up? Fortunately my client is pretty tolerant of time ranges, but I'm not sure just what I will run into after I do the actual conversion.