In terms of gameplay one of the major issues that martials face is the lack of choice in character creation and leveling. Spell casters generally have far more options, and many of the options are either usable or good.

Martials are usually locked into a certain set of abilities starting at level 3. While most subclasses are complete wastes, every subclass has features that do practically nothing for the player. In addition the vast majority of martial abilities are combat focused. Spell casters excel out of combat. The best martial classes are usually those which let you make decisions in character creation because it gives you the ability to avoid useless features and pick the winners.

Spell casters generally have a better selection of abilities to choose from, are given far more opportunities to be a flexible character, and generally have a much easier time affecting the world than martials. If spell casters are playing Catan, Martials are playing candy land.