It's an incredibly easy class. Easy to be good at, but also easy for new players to use. The only relatively hard thing to do with the class is rocket jumping and doing it well.
Let's check out the class real quick. ~6 damage per flame, which equals to around 1200. This is in the absolute best situation where none of your flames miss and at close range. The least amount you can do is around ~3 damage per flame, so around 600. All of that just for holding down M1 and barely needing to actually aim. Though these numbers are in the best situation possible, it still gives you a good idea for more realistic situations.
That's not all! You also have an airblast mechanic that reflects every type of gun that shoots projectiles, besides a very few, if not less than that, unique weapons. What else? Reflecting said projectiles and successfully landing a hit, regardless of distance between reflected shot and another player, results in a mini-crit.
For example, reflecting a rocket and hitting a soldier with it directly, at most, will deal 151.2 damage. The least amount of damage is 60.75, though this is the absolute least amount that you can deal from a reflect. Generally, you won't hit anywhere near the least amount and will probably score damage in the 100s.
Does this not sound ridiculous yet? Well, all right. The class also has a melee weapon that scores critical hits--not mini-crits like airblasting--when it hits a player suffering from afterburn. All classes suffer from afterburn, even the pyro, though in very, very specific situations. That's 195 damage, and you will always deal 195 damage, unless you're hitting someone not on fire (don't do that).
So on top of initial fire damage, then afterburn, then potential reflect mini-crits, you can devastate a juggled player with ease. However, what if you reflect a rocket and send that soldier in a direction other than one that allows for an easy Axtinguisher finish?
NO FUCKING PROBLEM.
You also have a flaregun, which deals 30 initial damage to a player that isn't suffering from afterburn, but if the player is in fact already on fire, well, there's an additional and instant 90 damage dealt.
So yeah, that's it. Welcome to the pyro class, have fun.