Well, it's still a T2.5 rather than a proper T3 for starters.
It's also one of the longest-ranged options for the Firebug, a perk that's mostly good at close-to-medium ranges. The Spitfire is another example that's far less expensive and lighter, but it's also NOT hitscan, meaning you have to predict a little where your target will go at longer ranges (granted though, the fireball travels fast enough so it's rarely a problem. You could say the same for the Husk Cannon). The Spitfire also has a ridiculous amount of reserve ammo!
The Incendiary Rifle is arguably better in almost every way. But keep in mind that it was added way later than the MAC-10. It's also a less favorable sidearm, which is where the MAC-10 excels IMO (although it's only true in the second half of a game, I can't argue with that)
It's another of those examples where the gun itself is perfectly fine. The main issue is that there are other options that are better. Making it less expensive probably wouldn't solve anything because the dragon breath is better (far more damage, including groundfire), and the spitfire does almost the same thing as the MAC-10 with added bonuses. It's only real advantage is obviously that it can headshot more reliably and deal with multiple targets faster. One could also be very ammo-efficient by choosing the "Barbecue skill", firing one or two shots at trash and let the afterburn do the trick... But it's rarely a viable strategy because killing zeds quickly is part of the meta.
So yeah, there you have it. In my opinion : it's a good gun, outshined by others which are simply better.
(And let's not even talk about the SWAT... It's totally a subpar option compared to the rest. It's only slightly more powerful than the MP5, which in turns is cheaper, has more ammo and equal ROF. And even that gun is slept on in favor of the MP7 or the P90)