Which theme are you using? Larry or Elastic?
Larry is indeed pretty old, but some do like it. The Elastic theme is the more modern one.
Check Settings > Preferences > User Interface > Interface skin to see which one you have enabled. If you've been using Larry, check out Elastic.
the list is not endless, but is limited to a certain amount of emails per page
True, there's no "infinite scroll" option, like in Vivaldi Mail, but you can increase the limit to 200 emails per page (the default should be 50, IIRC) from Settings > Preferences > Mailbox View > Main Options > Rows per page.
Also, if you compare any other Vivaldi site with the webmail one, you can see a pretty clear difference in terms of UI.
That's because we haven't created that UI. We're using the one made by Roundcube. This also means that we're rather limited in making changes to the UI.