Just for records. Tried to import from OperaMail with Vivaldi 5.4 M3
Tested import - it's rather clean and no stops as before. Very good feature to monitor log (accessible through envelope icon in status bar)
IMAP folders are imported (into custom folders), but they can not be reused as filters.
Neither labels nor filters (implicitly defined in labels) are imported (it was already known but only to precede the wishlist).
Here is the wishlist that will be also replicated in https://forum.vivaldi.net/topic/78072/advanced-organization-of-emails/6 (not sure if this can be made as feature request or so):
If, in the future one can import OperaMail or Opera M2 labels or only filters that define those labels (even without hierarchy - only to have their list and definition), it would be very welcome. Otherwise, it seems that filters have to be just redefined in M3 manually and if you have 10 conditions for one filter, it is tedious.
If there is any low level way to import labels/filters from Opera it would be welcome too - f.ex. just using index.ini file in opera to convert it to filters definition in M3. Btw, where are the filters stored in M3 and in which format?
Regarding request for hierarchical labels/filters. Actually, filters and labels (linked to filters) are rather parallel as concept and the only minor reason to use labels is that their tags appear on messages. So, actually, one can live with filters only and can conclude that for the moment if hierarchy is available for filters only, it would be sufficient and very welcome.
If hierarchical filters would be a feature, it would be great to have some functionalities to organize them - sort of filter manager (reorder, group, organize in hierarchy, change icon, search by filtering criterion, save messages belonging to filter, re-so search for whole group, etc.).
A more advanced filter management would be the ability to export/import filters individually or by group to migrate filters. Sort of "save all filters into a file".