Here's the question you need to ask all of the non-stock miniguns.
What do I get, and what do I lose for equipping this?
One of the reasons the Tomislav is so good is that it doesn't make the Heavy even worse at any of his downsides. Not only that, but the downside of 20% slower firing speed is almost completely made up for by the 20% extra accuracy in 1v1s. Depending on the distance, it can actually be more valuable than the firing speed. Meanwhile the other upside? 20% faster rev-down? Makes the heavy less bad at one of his downsides, the fact that he has to spin up to fight anyone.
The Natasha. Unlike the Tomislav you are becoming worse at something you are already bad at, your rev-down time is now an even bigger problem. You also become worse at something you are good at; putting out damage. However you also improve in two things the heavy is good at: taking the brunt of the enemy teams damage and teamfights. Heavy is already a class that is very hard to deal with if he has teammates backing him up. Heavy's poor speed and being able to chase down opponents can be perfectly fixed with a scout by his side, his reliable hitscan weapon and immense close-range DPS make it very hard for the enemy to get close enough to have an advantage against a friendly demoman, and a medic can completely negate what would otherwise be an extreme weakness to chip damage (like an enemy pyro spamming the scorch shot from across the map) among other things. The Natasha enhances all of this and helps the heavy in his natural job of helping the team.
The Brass Beast makes heavy very weak against two things he is already extremely weak in, in return for buffing his ability in two things he's strong at. The slower spin-up I've already went over how that's bad for the natasha, but the heavy is already the most immobile of the 9 classes. By a fucking mile. Remember how low on the totem pole he is, in 6s the medic is considered an immobile class, but he's still more mobile than a sniper, who is more mobile than a stickyless and shieldless demo, who is more mobile than a rocketless soldier, who is more mobile than a shotgun heavy, who is more mobile than a reved down heavy. And the brass beast reduces his mobility by SIXTY PERCENT? Mobility is something that is so important that it makes a 125 hp class that uses a shotgun with slightly higher ramp-up into one of the most powerful and devastating classes in the game. Because of fucking mobility. And the BB gun reduces the heavies mobility by 60%? That is just such a fucking strong downside in TF2. It takes the heavies biggest weakness and takes it to the extreme. 20% extra damage and 20% damage reduction are good upsides, but they are nowhere near good enough to make up for this. I can safely say that if the rev-down increase was removed completely, the BB gun would still be shit, but if the movement speed decrease was removed the BB gun would be flat overpowered.
The Heater, at least. It again makes the heavy worse at one of his downsides; he's bad at conserving ammo. The problem is the upside it gives you? Isn't an upside. The heater is inherently flawed as it only would improve a new heavy's abilities. Anyone who gets that close to a heavy is already going to have to deal with his 400 DPS hitscan minigun. Only if a heavy can't aim will the heater actually come to be useful, otherwise anyone would die before it managed to get anything done. The Brass Beast may be terrible, but it's at least got a niche and is fun on pubs, the Heater is just a direct downgrade to stock.