Here is the first issue. Number of bodies doesnt matter anymore to tyranids. Thats why I dont like them in their current iteration. They are described in books as massive amount of fangs and claws who chew their way through the enemy with a few big leaderbeasts to take on the hardest enemy resistance.
How they are currently working? A lot of huge beasts supported by a few small shooting beasts to unlock another huge beast. Currently the Tyranids are a race of a few monstrous creatures fielding sometimes even less creatures than an average space marine army. Imperial guard vastly outnumbers nids when it should be the other way round.
Yes, every shot that is aimed at a pyrovore is not aimed at another big bug. 100% agreed. But every pyrovore squad means also LESS big bugs. (a squad of 3 is another carnifex/harpy/biovore squad..)
The deal with the first part is: Any unit that is capable of "countercharging" deep into the tyranid deplyment zone probably ignores the flamer and just shoots the pyrovores to pieces.Second: I think what hurts the pyrovores the most is its SLOW speed and short range. Sure those heavy flamers are great, but you have to get in range to use them. So pyrovores might be best as defensive interceptor units... midtable support and control against aggressive lists might be the best way to go. If you really really really want to get them across the table and in your opponents face, I think a good option might be trygon tunnels with an aegis com-relay. Its a bit of a crap shoot, but with some luck, you can get a trygon tunnel forward on turn 2 and your squad of pyrovores in the opponents backfield on turn 3. The more Trygons and pyrovore units you have, the better your chances are, but that is quite a heavy investement.
To the second part: Yes. May work. Heavy investment and if you mess up your reserves the game is basically over. But yes. The question here is more of: If you invest that heavily into this one trick pony, why not pick units that can do the same job cheaper/better?
Not. Some units are not that bad. But there are obviously way better choices which make them seem bad (unless all better choices are extremely overpowered which probably not the case with nid units).Last thoughts: Some units are definitely NOT as bad as the internet makes them out to be. Most people's opinions are based on theorycrafting and not on actually playing with a squad of pyrovores. Give em a try (proxy them in if you don't want to pay the steed price) and see what happens. Also remember that using them just once is not a very good statistical sample to determine their actual value. Play a few games and try out different combos to get a more accurate feel for them.
The problem with pyrovores is that they come in a unit of its own, have a rather cheap weapon and you still pay a premium price for them.
However I can imagine that they could be uses against orcs to a certain extent. But again as suicide unit. and paying 40 points for a suicide unit is possibly rather unwise.