D&D 5E How well can your players and NPCs maintain concentration for spells?

[Zardnaar]

A downside to the concentration rules we are discovering is that the cream is rising to the top so spells like resist energy which require concentration are not getting used.

 

[Riley37]

Have you ever run a master villain who has multiple minions, each minion maintaining one low-level Concentration buff?

Do the minions need line-of-sight or some such to maintain the buff, or could they be even farther back than "back line"?

I've considered one PC maintaining concentration on their own Invisibility, while another PC casts and maintains Hide the Trail on the invisible PC. It could be a powerful synergy.