Fighting games often get accused of not evolving enough with new iterations. New balancing tweaks, a few new characters, new movesets for old characters. Ultimately, a new entry in a fighting franchise is judged by how it stacks up to its predecessors and whether or not it moves the franchise in a positive direction.
Soul Calibur, one of the first successful 3D fighters, has had a tumultuous history. Always respected and embraced for its addicting and deep blade based combat, where Soul Calibur has struggled is in delivering consistency in its story. To understand the story properly, multiple flow charts threading the array of characters would be needed just to establish relationships. Throw in the problems that arise when canon (anything involving Siegfried) and non-canon (Zalsalamel ending up with Soul Edge after 3) stories collide resulting a trip to a bookstore to pick up a copy of Soul Calibur Cliff Notes.