Well you are right about that, Blood Magus is a OP start of your run but for Rogue it needs planning and tactics with the knowledge you obtained after you played many runs and know the layout of the gameplay like for example in TOB it is your starting for Rogues while it will be quite a difficult start from scratch, They must obtained fables at the start in the docs in order to overcome its disadvantages giving you a boost whilst giving you a negative effect at the same time.
So in short
Blood Magus are OP in the start if you build and save enough equipment for upgrades as they can simply Cast Chaos throughout the gameplay with ease
While the Rogue on the other hand they tend to plan and strategize on what to do next and create a suitable skilltree for them, High risk comes with High rewards, There are some fables grant tremendous advantage while disabling some skills or some restrictions, it is up to them what kind of gameplay they want to play in their run, when they get used to their current run they will get much stronger and will likely rivals the Blood Magus eventually.