Forbidden Arcana is written so that the GM has rules - simply put, using our general statement that 'the rules apply to everyone', they've given general rules so that the GM knows WTF 'is going on', as it were. Whether the GM wants to allow the PCs to use those rules is up to him; typically, the answer for a lot of that (e.g. blood magic, toxic, insect spirits, shedim) is going to be 'no'.
Rules for such things - for mages using such things - has been in existence since at least 3rd Edition, and 1st if you want to get technical about it. Because of the way 5E is being written, however, it gets presented as 'this is what we know, this is how we know it, this is the viewpoint from someone who does this crap' - which means that it's written from the 'used by a PC' viewpoint.
While there are failures-to-launch on a few things, overall I understood perfectly that most of the areas in Forbidden Arcana Firebug is working out his blood pressure over were to be read as being 'these are all options, but if your player wants to go there, you'd best think long and hard about saying yes to them'. When you get down to it, it's a flavor preference in regards to writing style and presentation. Because there are always going to be people who want to know the rules for blood magic, or toxic, or etc. God knows we did back in 2E, when all we knew was 'Druids in England use GOOD blood magic, Aztlan mages use BAD blood magic', and we had no way of knowing specifically what the difference between the two was. Now we have something that is very clearly hammered out and put up for us to see.
In regards to most of the other issues that Firebug rants about (such as the Focused Awakened and the various Traditions), as I read it these things are optional slants for full mages or mystic adepts; give up these things in order to get those. Because of the balance between the advantages and the disadvantages (and if you think that the Drake Tradition's having to learn every new metamagic from their Dragon Overlord is not a disadvantage, you're definitely playing some variant of the Shadowrun world) of such things, IMO they're a wash on the 'purchase' point - but perhaps those should specifically be addressed ...