Drupal 6 Ubercart products nodes migration to drupal 8 ubercart ?

Moving this to the ubercart issue queue.

Upgrading from D6 to D7 is a simple process that works very well for Ubercart and most other contributed modules. Given your recent posts, I would strongly recommend that you first upgrade to D7 as that should be a short and relatively easy process. The biggest problem would be if you had some D6 modules that weren't ported to D7, then you would have to port them yourself or hire someone - that problem is 10 times worse if you're trying to go from D6 to D8, as the majority of modules never get ported to a new major version of Drupal.

Core Drupal 8 no longer has a simple upgrade path - you must use the Migrate module, but the core Drupal 6 migration path to core Drupal 8 isn't yet working, and wasn't planned to be working until core Drupal 8.1 is released. As far as I can tell, even then some major bits are going to be missing. And even if you manage to get core Drupal migrated properly, very few contributed modules have implemented a migration path yet because all the Migrate stuff is essentially new as of a few months ago.

Additionally, D8 is only recently released and many modules have not been ported yet. Core Ubercart for D8 works and is in pretty good shape, but most contributed modules (like payment methods etc.) have not been ported yet. I wouldn't try moving an existing site directly from D6 to D8 at this time unless you're prepared to do some serious development work. You should upgrade your site to D7 first.

Core Drupal 8 no longer has a simple upgrade path - you must use the Migrate module, but the core Drupal 6 migration path to core Drupal 8 isn't yet working

This isn't true, the D6 to D8 migration path is both a simple replacement for the upgrade path and has been working for over a year. Sure the UI wasn't in core but you can still run it with the drush commands from migrate_tools or the UI from migrate_upgrade in 8.0.x and the migrate_upgrade UI is now in core as of 8.1.x

@benjy: Well, I have to disagree. And whether or not I'm right is irrelevant to my answer - the OP isn't a programmer and therefore wouldn't be able to handle a complete D6->D8 migration at this point in time because even if you do think it's simple it's not complete and WILL require custom plugins to migrate anything other than the most basic core data. Whereas the OP, and anyone else, can easily do a D6->D7 update with only minor difficulties that have been well-documented in the issue queues over the past 6 years since D7 was released. That's the whole point of my answer. Try not to take it personally - you may quibble with the details but based upon many posts the OP has been making over the past few months advising him that it is simple to migrate his site from D6 to D8 is extremely poor advice.

Most Drupal users are non-programmers who don't have full control over their webhosts, therefore can't install composer and drush and possibly don't even have a command-line to work with. What's 'simple' for someone who has a lot of familiarity with Drupal internals and can program is an extremely intimidating task for most users, and most people don't have the tools to overcome the many obstacles that will be encountered when migrating a D6 site to D8 at this point in time. I count myself as part of the experienced group, and I certainly wouldn't attempt a full D6->D8 right now unless someone wanted to throw a lot of money at me. Even then, I can think of a lot of others things I would rather do. If you would like to sit down with me in IRC and walk me through writing a custom migration for some Ubercart data, I would be more than happy to find out I am wrong in this regard.

While there was major work done in Migrate in the fall in the few months before the 8.0 release, before that there was essentially no D7->D8 migration. And there are still things missing like Views migrations which I guess you can argue aren't really core migrations but without them, well ... Just having core migrations isn't good enough and while an incredible amount of work has been done with core migration it just wasn't ready when 8.0 was released (and wasn't targeted to be ready for 8.0 either...) and is still in flux right now. Writing migrations for contrib, especially for complicated contribs like Ubercart, is difficult because of lack of documentation and examples. Again, it's much better than it used to be, I don't want to discount the hard work that's been put into it, but what's available is really not sufficient yet. I have some migration stuff that was written for D6->D7 but it's basically useless and has to be rewritten because there's no migration path for migrations that I know of, and no documentation I've found which shows how to port migrations to D8. In contrast, upgrades from D6->D7 were done via hook_update_N() which worked very well and were easy to maintain as modules were being developed - every time you made a data model change you could put the change in the hook and it formed a self-documenting piece of code for contrib which showed how to get from the old version into the new version. We maintained update hooks for Ubercart D7->D8 for over a year until the rug was pulled out from under us and that mechanism was replaced by Migrate (which BTW didn't really exist yet when that happened). Then we had to wait another year for Migrate to get to a point of usability and now again there's talk about refactoring which will break BC.

When Migrate is ready and finished and, more importantly, well documented, and when contrib has had a chance to catch up to the major work that has been done on Migrate over the past few months, it will certainly be a more useful and capable tool than hook_update_N(), but it's NEVER going to be as simple as going to /update.php and pressing a button.