What these guys said. Being on fire is a big deal, and my favorite tool for scaring PCs of all power levels. Even a Space Marine or Ork can't fully mitigate the effects of fire. Being on fire is really really bad.

That said there's ways to mitigate that. These guys already gave you the most obvious stuff, burning stuff that's important(the tapestry of Saint Murdicus!!!!!!) and range. Tossing in enemies who mitigate the dangers of fire, either because they're really slippery or they're really tough. But consider this, have a bunch of low level enemies close into melee with your characters, form a broken line where it's impossible to open up with flamers without friendly fire(ha).

Flamers are badass, there's a reason beyond the ceremonial why they're a semi ubiquitous weapon in the setting. There's also a reason why they're situational niche weapons and not considered the go to weapons like bolters are. They have terrible range, their damage doesn't scale, there's not a lot of fancy attachments you can do with them, and their to hit doesn't scale with skill. If you're a badass with a bolter, you can tag one dude 4 times, the chances of him dodging all 4 shots is pretty low. The chances of him making one of two agi tests is a lot higher. This sorta leaves flamers in the same piles as the eviscerator a powerful niche weapon, that's good but not optimal, best used by those with meager funds and lower characteristic scores.

That said exterminator cartridges on everything is a borderline standard with many of my players. A one shot free flamer tied to my bolter, thanks! To me the only thing better is a combi melta, but that's for a whole other power level altogether.

There are some guys coming up who have flamers as one of the users here pointed out. Fortunately, my crew haven't figured out combi weapons, so I've been blessed in that way otherwise, I'd have an entire squad of guys with flamers. They have figured out rounds that cause a target to be on fire though, but so far I haven't seen many of those.

Does anyone know the effective range of a real-life flamethrower? I think the last time they were used in combat was WWII in the Pacific, and that was at very close ranges to clear out bunkers and clear vegetation.

RAW, a heavy flamer can break 100m at extreme range, which is really freaking far. Crazy far. Unless there's some rule I'm not seeing that prevents flame/spray weapons from exceeding their printed ranges (which would be a great way to curb their overuse).

e; Vietnam, but same use case

The general average for WW2 models is 30-40m. Modern militaries generally don't use the classical flamethrowers per se anymore. They're far too cumbersome for something you can do nowadays with a grenade.

Modern incendiaries and flamethrower variants...vary. BMW has a car security one that only reaches 2 meters, while the Handflammpatrone, which is more a short range incendiary rocket/flare, travels 8m through the air, then explodes and blasts everything in a 15m wide and up to 90m long path from the point of impact. Unfortunately, it was decommissioned in 2001 and the Bundeswehr no longer uses it.

Then you have "civilian" models like the XM42 that only shoot 25 feet.

Some vehicle mounted models reach the 100m mark as well. I think the Brits had one for a while. Wasp or something. Not sure if it's still in use.

So, given 40k plays in the far, far future, I would have little issue with over 100m ranges, if they actually appeared in fluff. They don't, though, so I'll just point to WW2 models go "nope".

Unless there's some rule I'm not seeing that prevents flame/spray weapons from exceeding their printed ranges (which would be a great way to curb their overuse).

p.149; Spray Quality.

Unlike other weapons ,they have just one range and, when fired, hit all those in their area of effect.

I made these templates for use in roll20.

http://imgur.com/a/xYsgu