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 "Lončar" 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 Ä, they fix Ä ... 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, ä->a, ö->o, ß->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.