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.