Opera 12.x notably was great browser with full feature in one package browser you can get back then compared other browsers and some features was ahead at that time.
I did use Opera Chromium-based after Opera drop support Presto engine and it was ok but I switched back and (after couple years later) some website stopped working, intrusive certificate error, and opera sync stopped working.
Last week, I gave a try again using current version Opera, and it runs great, but some features missing such as widget on speed dial, progress bar, seriously customizable panels and speed dial make it as yours, and lastly tab grouping.
And Tab grouping is the most feature i really wanted to be back, it was so ahead from that time managing tabs as easy as drag n drop.