I tried QtWebEngine 5.6 on an Ubuntu 14.04 Trusty base (JuLinux) and a Debian Jessie (MX-15) base. QtWebEngine is a port of the Blink engine to Qt. I built the QupZilla, Otter and Qt Demo Browsers against it. '

On, Ubuntu Trusty, I couldn't connect to Youtube, Google, and Twitter due to an SSL connection error. I also couldn't get Pepperflash to work even though it is supposed to work beginning in QtWebEngine 5.6. In Debian Jessie, I was able to get those sites to play, and I could get Pepperflash to work but only in the Qt Demo Browser. I wasn't able to get fullscreen to work when I played Criminal Case on Facebook.

All in all, QtWebEngine looks promising. It is more secure than QtWebKit 2.3, and QtWebKit 5.x. But, its best used on newer systems, and not used to play flash games. I'd still put it in beta quality but Qt Demo Browser, and Otter are candidates for default browsers on Qt centric distros, although many users will still want to download Firefox, or Chromium. Personally, I prefer to use SeaMonkey or Pale Moon to browse.

I just tested QtWebEngine on Xubuntu 16.04 Xenial beta. I built Otter and Qt Demo Browsers against it. I can connect to Twitter, but not Google and Youtube. There is the same SSL connection errors. I am able to get Pepperflash to work in the Qt Demo Browsers and play Vimeo videos fullscreen. The game Facebook game Criminal Case wouldn't load. Pepperflash doesn't work in the Otter browser, but the Otter browser was designed to be built against QtWebKit 5.

There should be a high priority to fix the connection issues on the Ubuntu base, considering its the third most used OS base on desktop computers.