When it comes to minions, the mixed-level party was probably the biggest issue. I ran a game with, IIRC, 10th level PCs supporting defense of the town. The town had it's own guards and using the conventions of 4E I normally would have been using minions for the amassed horde. But I couldn't because then those low level guards would have been 1-shotting monsters that should have been a significant threat.
There are many ways to handle monsters that are significantly lower level than the PCs, in 5E the standard is to set them up as mobs. I greatly prefer the 5E approach.
There are many ways to handle monsters that are significantly lower level than the PCs, in 5E the standard is to set them up as mobs. I greatly prefer the 5E approach.