It ended in copy & paste 400 articles ourself manually

By the occuring bugs I found out, that they count the letters in each text paragraph by script …. but obviousely wrong. This counting method is used to set formats like bold/italic or the position of inline links. Becasue their method obviousely only supports plain ASCII chars or one-byte UTF-8 chars (one of both, not exactly sure) and counts wrong on ANY “special” char like the tripple dot, you only need to have a cite of a polish person with a name like “Lon?ar” to mess up your text. Citing in general is dangerous, as also quotation marks counted wrong.
Be aware! There’s not only an offset, means a misplaced format or link, we found cases where a paragraph was missing completely or the order of paragraphs was diced.
Their support only tries to fix it cosmetically … so if you tell them there’s a problem with the letter ?, they fix ? … but not the problem in general!
We used the migration also for a different thing … we paied extra for SEO resons the 301 redirections. As this Wix Software support for example German umlauts in the URL (non standard), but WordPress replaces them correctly (ü->u, ?->a, ?->o, ?->ss) also this part of the migration failed hard ….
Asking for a discount, becasue their migration process is total crap resulted in “We are support staff and can’t give a discount”.

So if you use ANY char in your URL or Text what doesn’t fit in [a-zA-Z0-9] then be VERY carefull and compare EACH paragrapf and EACH permalink!
If you have ANY language involved that is not english, it will fail with a probability of 100%!

To add on … it was NOTHING complex …. just a news site with mostly one picture per article, some cites, some links … only the picture part was done without any issue.