I understand that that is your opinion
but its a rather narrow minded one if you ask me. Yes the voices table will help get possessed into combat faster with greater numbers but they then lack the ability to be as useful. Vessels might mean they are slower but they can rip more units apart because of it.
Honestly, in my opinion there are a lot of ways you can go in favored of chaos builds, I personally like the idea of running a mixture of Slaanesh and Khorne possessed around a Tzeentch demon prince.
Though I do agree with you, it really does need to be paired with another formation or detachment because there will be quite a few shortcomings that they cannot make up.
Uhm..is that what the Ork and Space Marine players regularly bring to the tables you play? Because I haven't seen hordes of those kinds of armies out there. Space marine armies with two or three grav weapons and another two or three melta weapons sure, and 40-60 shoota and slugga boyz yeah, but certainly not a lot of lists with 10 meganobz that couldn't be easily countered in and of itself. (I mean thats 400-500 points without troops or an HQ or to many other upgrades right?)
A formation that costs more than 600 pts for 15 infantry + 1 MC has literally just one way to be played: the one with the more bonuses involved. Otherwise we might just use something else...as narrow minded as it can sound, i want to have fun, but i also want to win a game here and there. The formation is a bet against myself, if i play it. Because possessed are an handicap (pointwise) if you field them.
And no, that is not what SM and orks bring usually to the table, but that exatly my point: even poor choices like those can rip the formation apart, with half the points involved. the whole formation is just shredded by 2 units of 20 shoota boyz, that are half the points. The math is simple: even if you have T5 and 3+ armour or invuln, you still have a 1/9 chance to de from a bolter shot that hit you. And that is all. They might be awesome, "hard hitting" (man, have you see the wulfen shit??) and cool, but they cost twice wat they should to be mathematically viable.
Again, ok, cool, nice post and analysis but please lets not glorify a turd because it has a cool name (Favored of Chaos) and lets not think to be open minded because we consider good something that ise generally despised for a good reason...