@pala62, I believe Opera will never "switch back to the old Opera 12 engines", for several reasons. First, they moved away from developing the underlying Presto rendering engine because it was too costly and inefficient to maintain in a world of fast-changing web development dominated by sites catering to big-browser engines. Second, too much time has elapsed and money has been spent in migrating to the new engine to simply write it all off with nothing to show for it. Third, some of the major Presto designers have moved on to other things in life... and it can be an incredibly daunting task to go back several years and successfully pick up all the strings of a development project that was completely halted at the time.
Until some new security flaw emerges that Opera chooses not to patch in Opera 12, there is no "good" reason (other than slowly growing site incompatibilities) to not continue using Opera 12. "Officially', Opera has said they are still supporting Presto Opera from a security standpoint (which is how 12.14 evolved to 12.17). Once such support officially ends, continued "safe" use of old Opera versions would depend on a user themself staying abreast of emerging security threats (at sites like Secunia, CERT, etc) and taking whatever action they could, if any, to mitigate the emerging threat on their own. Personal browsing habits (including browser and OS settings) will, as always, dominate the safety of one's online browsing, regardless of the browser used.
As far as which version has the least bugs/issues, I'm not sure which Opera family you're referring to. Either way, determinations about bugs and issues are strongly personal, and YMMV. As far as my own viewpoint, in Presto browsers, the "sweet spot" was Opera 12.14, largely because later versions had a "locked" default-search-engine list (similar to the new Blink Opera versions), and those choices did not include my preferred StartPage choice. Over the years, other favorite Presto versions strongly preferred by users were 10.53 and 11.64. The older the version, the more unpatched security flaws it contains, though security-through-obscurity does come into play - particularly for small-marketshare browsers represented by the old versions. Finally, some users swear by 64-bit Presto Opera versions, while others swear at them... so again, YMMV.
With regard to Blink Opera versions, IMO the latest-is-best philosophy applies, for several reasons. First, the new versions are designed for auto-updating with no manual update-blocking choices (unless one does that in the OS or by file-fiddling). Second, because the Blink rendering engine is shared with other browser brands and derived from Webkit, it represents a much more attractive hacking attack-surface than did old Opera's Presto engine with its limited user-base. Consequently, it's much more important to keep it well-updated with new Blink releases that incorporate new security (and bug) patches. Finally, the new Opera versions are evolving steadily (bookmarking being the most evident recent area), and such improvements are being implemented incrementally with new versions.