Flamethrower is, in my opinion, the best Guardian XP card by a large margin. Of course, I'm hardly breaking any ground with that statement. But I can't resist writing down in just one more place how amazing this card is.
A while back, I wrote an enthusiastic review of Lightning Gun, saying that 3 damage per action and massive bonuses to Combat were some of the most important tools for success on Expert. Well, Flamethrower outclasses Lightning Gun in almost every way. 4 ammo over 3 ammo is very nice. Costing 4 resources instead of 6 makes a huge difference and really accelerates your opening setup. But 4 damage per action instead of 3 is the real win. There are a LOT of 4-health monsters out there, and this card makes them seem pathetic. These are the most powerful enemies the game has to throw at you, and with this card you trash them in one action and wonder what to do with the rest of your turn! Vicious Blows start to rot in your hand, unused and unneeded. It's ridiculous.
And I haven't mentioned the upside of sometimes roasting two or more monsters in an action. It won't happen every scenario, but it's very nice (and very fun) when it lines up.
You also get very good skill icons. I don't care that much--I would play this card if it had no skill icons at all--but it does make for a nice combo with Well Prepared.
You do lose out on some to-hit bonus compared to Lightning Gun, but +4 from Flamethrower is still awesome and will get results on Expert.
You also lose out on a body slot. This means you can't run Bandolier. But I didn't like Bandolier with Lightning Gun anyway, so that's no great loss to me. If you have a well-stocked Flamethrower, you really don't need other weapons. You lose the ability to run a Flashlight or the like. In solo, that does matter. In multiplayer, I don't care about that at all. You'll help your teammates far more by efficiently roasting all enemies with Flamethrower than you would by investing in some dinky investigation asset. (Of course, you do want to have some other assets in play, so you don't lose Flamethrower to some Crypt Chill effect. But there are plenty of good options that don't take up hand or body slots.)
As for the "must be engaged" requirement: not a huge deal. Most of your combat was against engaged enemies anyway, the massive action compression of Flamethrower more than makes up for the odd extra action you might have to spend, and Guardians even have some nice little events like Taunt and "Let me handle this!" that can help out if you feel it's needed.
A good way to build around Flamethrower is to get Stick to the Plan, and attach a Prepared for the Worst, an ammo card, and an economy card to it. Another very good support card is Beat Cop II, which not only helps your Flamethrower attacks land, but also saves ammo and actions by taking out small enemies for you. And I would be remiss if I didn't mention Double or Nothing...