I've only played on Tactician and Honour Mode.
In the Honour Mode runs I failed, I deleted the save because my only reason for playing the mode was for the achievement. As for what killed me, usually it's stuff like random crit at an inopportune time or failed spell save leading to catastrophic results (LIke having a strong character get mind controlled in the Nere fight and instantly murdering a party member)
As far as the most fun, I like Honour Mode's inclusion of Legendary actions and some encounter changes. But I hate single save...
So I had more fun in Tactician (Also, the fact that a majority of my game time was before Honour Mode was even a thing so Tactician was the most difficult setting possible)
There may be custom settings for Honour Mode + Multi-save these days, I dunno since I haven't really had the desire to play since finishing Honour Mode (As I'd already played out all the different class/race combos I cared for by that point)
