What this says to me is that ARM-64 is not considered stable yet and hence only available in unstable (snapshot) builds. Possibly they just don't have enough testers yet, though you could ask @Ruarí about that. (As far as I know, he's the main guy in charge of Linux builds.)
They've already mentioned they're working on an extensions interface (the ability to accommodate certain extensions is already inbuilt, and I have already installed one of my critical ones) and so I shouldn't think at least early extension support is more than a few weeks down the road, since they've provisionally committed to releasing a build a week. I would guess the two elements their mature support will have to address first will be signaling the Chrome Store that the browser is, or is like, Chrome, and then equipping the browser itself to differentiate and tell the user whether this or that extension in particular is actually supported.
Why would you like to go to google when Vivaldi starts? Just use either the URL-TextField or the search-field next to the web address whenever you need to look up something. For me this works ways better than a start page.
But I aggree, that an option for Browser Startup is missing (not for me but for a lot of others)
Finally I found this 'workaround'!
On about hundred websites the download link is 'https://vivaldi.com/#Download' wich leads to 'https://vivaldi.com/download/Vivaldi_TP2_126.96.36.199.x86_64.rpm'. Which… doesn't exist.
It's a bug. It's been mentioned so often so many places since this build was released, the devs have to be aware of it by now. I'm sure they'll address it.
We are fully aware of it!
I just installed latest .deb package of vivaldi on my lubuntu 14.10 amd64 and i have two vivaldi browser in system: previous, technical preview build and current one, labeled as "unstable".
Unlike other browsers i have installed on my OS, vivaldi can't detect mouse 4th and 5th key which i'm using to go to previous/next page, browser doesn't react to these keys and is not switching pages.