I guess we should start from the top then. I figure this is pretty basic stuff, but it's best to get everything down so we're on the same page.
Flamers can do lots of things. They aren't as good as Fiends, nothing is, but in a mono Tzeentch army you make sacrifices for the sake of fluff. Not my bag, but people will do as they like.obviously- your tactics for mono-tzeench show you dont like them... meanwhile I love mine and have taken it to tournies before now: its never going to win but its a fun army that can compete with almost everything What they do offer is mobility and versatility. They can fight from 30" or in flamer range, which is useful against a variety of opponentsthey can't fight from 30".. they can start 30" away and do damage but they'll be <24" if using bolt and close enough for the enemy to move 6" and get into RF range if you use warpfire, for flamers that means game over.. If all you take is 1 unit of suicide Flamers, then you will of course be limited by that perception. 1 unit is fragile, but what about 3? 3 and 4 Heralds? 3, 4 Heralds and 3 DPs? 3, 4 Heralds, 3 DPs and a bunch of Horrors? They all have bolt, they all have alternate fire, and they are all threatening. This is how a mono Tzeentch list works, if at all. Present so many targets that your opponent cannot possibly handle them all. 1500pts for those units and the enemy can kill the 400pts of flamers with ease as soon as they look like becomming a problem. Sure you have some anti-tank but if everything in your army fires at AV11 enemies then you'll get about 4 pen hits and 1.5 glances... so you destroy 2 rhinos, bash up 2 more and havent fired a single shot at any of the enemy infantry... whats worse is that next turn (assuming the enemy infantry havent just erased your army) you now have 2 less enemy transports around but have to start all over again since only the DPs have the strength to destroy any of those damaged transports.
1 bolt isn't going to destroy a vehicle. Hell, 3 bolts may not. What they will do is shake, stun and immobilize them. A vehicle not shooting may as well be dead to you for a turn. It works a lot better when you have actual combat units that can take advantage, but Mono Tzeentch makes up with the greater concentration of AT fire and anti-infantry follow ups. Against mech, you suppress the shooty and crack open the transports so you can warpfire/gaze/breath them away. I'll let you into a little secret- when I play my mono-tzeentch mech lists are what I want to see. Tanks are not a threat to tzeentch, infantry is the big problem. Use bolt to damage or destroy the transports, warpfire and gaze to kill anyone who falls out and then race in with screamers to surround the damage transports to stop people getting out and/or to destroy them either that turn or in the neemy phase- they then become both the closest target to the enemy and the most threatening... so you almost everyone shoots at the cheapest (and 2nd hardest) tzeentch unit in the dex.
The problem with Screamers is that they don't do the flexibility thing. They have no alternate purpose. Mono task. Thats pure bias- they have at least 4 major uses in my army, one of which is combat. They have WS 3, T4 and a 4++, that means that while they can only hold a small unit of marines in combat for 2-3 turns 3 screamers charging a unit of 10 guard equivalents should not only survive but should win the first round... and then hold them in combat for a turn or 2, meaning you can concentrate firepower on another unit without having to worry about the 20-30 lasgun shots raking your flank Taking them over better units makes target priority much easier for your opponent. I lose 1 unit, to get 3 of screamers... thats not a great loss, even if the enemy want to ignore the 3 extra units flying round... which would make me a very happy panda if they did He can simply focus on the important stuff that scares him now, then circle the wagons and move 6.1" to let the Screamers whiff on some dinky transport. Then he kills them. Kills them with what- he's ignoring them remember... and while he's ignoring them my 50pt unit just made his transport move fast enough to not be able to shoot or to get supporting fire from inside... and I still have a ~42% chance of getting a penetrating hit, a ~14% chance of destroying the vehicle that turn and a ~14% chance of stopping the vehicle moving so I can kill it next turn An army that picks up screamers is throwing points away, points better spent on actual good units like Fiends. fiends compete for space with flamers and BCs, which some people like to take. Screamers are the best FA unit by a mile... not that anyone ever runs out of FA slots unless they are screamer fans. A full selection of screamers costs 144pts, so pretty much 1 HoT, 1 simply DP or 1 flamer unit needs to go to make room for them (if you did drop a fiend unit for them, not that I would, you would still have 560ts left over from the 1 dropped unit after getting all the screamers you'll ever need They are usable in a mono Tzeentch list, one that's already filled up it's FOC with units that are useful. They're a poor substitute for the stuff you really need.