These are just a few notes explaining the tier rankings and other considerations for martial initiators.
Dreamscarred Press published Path of War and a number of supplements supporting initiators, but initiators were first-party in D&D 3.5 (published by Wizards of the Coast) in Tome of Battle. Initiators learn manuevers and stances from a number of disciplines, which function much like spells (but are extraordinary in nature). These characters remain popular, but there has been noticeable power creep between D&D 3.5 and Pathfinder. This can primarily be attributed to the supernatural disciplines, which bring maneuvers even closer to spells in function and scope.
As a result, I've decided to rate initiators with access to two or more of these supernatural disciplines as tier 2, those with access to only one as tier 2.5, and those with access to none as tier 3. A supernatural discipline is any discipline for which all of its maneuvers are supernatural. The Tome of Battle initiators are classicly viewed as tier 3, but they possessed few supernatural maneuvers. Some Path of War initiators require access to at least one supernatural discipline to function, and these tend to be quite strong. I've found that modern initiators can go toe-to-toe with other tier 2 characters in combat, maximizing their action economy. Tier 1 spellcasters can still outmaneuver them (usually by avoiding combat all-together), but in a practical sense combat is easy to force, and the current tier rankings reflect practical, not theoretical, optimization potential.
To offset this somewhat, I'm open to players crafting custom martial traditions. This way, players can still play the initiator class they want as a tier 3 class, and still gain access to dozens of disciplines between Tome of Battle and Path of War. Assume that maneuver granting items (such as those Tome of Battle), will be made available to all disciplines (but again, these count as accessing supernatural disciplines).