We recently switched back from Rapid Board to the Classic Planning/Task Boards because the Plan Mode is simply not useful for our needs (yet).
Our main problem with the plan mode is that we cannot see how the workload of selected issues is distributed (like, does one developer have 20 issues with 4w of work while another only has 3 issues with 2d), in order to make sure that noone is bored nor overly stressed during the sprint.
The classic Planning Board allows this to some degree, as it shows all Users at the side when switching to the Assignee view (which is still a bit problematic, as it shows all users, not just the ones that have tasks and/or are part of the project).
What we could use there is a view similar to the Planning Board Assignee view that comes up when selecting issues - one that shows all users with tasks for the planned sprint, aswell as a summary of how many issues and/or how much estimated time it should be (and a really nice-to-have one: set weights for part-timers/externals and constraints or some other sort of visualization that indicates a high workload compared to others).
But, until that happens: How are you handling this with the current Plan Mode (GH v6.0.1 as I'm writing this)?
Note that we have to deal with specialization, so we cannot just go and use unassigned issues, nor have everyone deal with any kind of issue.
