I use Mono and coated runninglines, depending on fishing needs and approach.
From a technical standpoint additional strength, longevity and the opportunity for closer tolerances in production are always advantages of a full cage reel. No matter which sort of runningline is used.
There may be reels with less weight, but for speyrods itís not an issue. If so, itís a problem in the troutspey spectrum with stupid downlocking reel holders.
For SH rods I used different open cage reels and came away from them. But they all were not in the high end spectrum.
After that I never used anything else for DH rods as full cage reels. Even with those I made bad experiences with some manufacturers because of issues with runninglines caught between cage and spool.
In the end I would say, itís not only a question of the construction principle, itís also to a big part the quality in production.
Time is running... A little bit of work, a little bit married, fishing, hunting... What else?