I previously wrote about my changing use of Day One here on this forum. In essence:
- Sync proved notoriously unreliable if I edited multiple old entries on my Mac (they either didn’t appear in edited form on my iPad or disappeared completly).
- Search in Day One was, and continues to be, very poor and ridiculously unsophisticated. That was a significant driver for me to export 17,000+ entries (in markdown format) and import them into DEVONthink. I wrote a little about that here on the forum.
I now make all my daily diary entries in Day One on my Mac, use hashtags for tags (which are automatically converted to DEVONthink tags on import), use frequent cross-references (links to other entries—which I convert to DEVONthink item links on import)…and, above all, enjoy the brilliant DEVONthink search facility which enables me to recall, in seconds, any item from over 50 years of writing a daily diary (now well over 19,000 individual daily entries). 
In summary, Day One is pleasant enough for creating the entries (so long as you don’t use sync) but DEVONthink is streets ahead for actually making use of them.
Stephen