The Duplication is one problem I encountered, Another one (in Opera) is that a tab's own history after closing and re-opening is lost now (maybe that's what you meant with the history), which is not happening in Tree Style Tab for Firefox.

Additionally, in Opera, all tabs are shown in the tab bar, which maybe an Opera-specific problem. With a few hundred tabs that can get pretty dense (and was not the case earlier, when sidewise was maintained). But I am not so firm with the "deeper" configuration possibilities of that browser.

Really ? No Answer within a week ? - I use Sidewise in Opera for several years now via the Chrome Extensions addon, but there are some annoying issues with the new Opera Versions.