The mafia setups nowadays are extremely similar. They tend to go something like this: 3 mafias, a neutral killing, then two weaker neutrals then a balanced town with a variety of different roles as well as a guaranteed doctor and sheriff.
Generally, the random function in the game fits pretty well. All of the roles in a type of random will tend to do a similar function and will have a similar level of power. For example, all of town investigative will be relatively powerful (exc. sheriff) and designed at catching evils.
However, the neutral killing random (which nearly everyone uses in their saves) is extremely unbalanced. Seriously, look back at games in your replays. And you'll find that Arsonist will rarely actually kill anyone meanwhile the serial killer will snipe someone every night he's alive. Right now arsonist is extremely underpowered and Serial Killer is extremely over-powered (in terms of who will kill more people, not which is more easier to win as). What makes it worse is serial killer, arsonist or mm cannot be excluded from the randoms (apart from neutral evil). This means the neutral killing has a wide, wide range of power inside (in terms of how many towns will get killed NOT ARSONIST BEING UNDERPOWERED PLEASE NO MORE, NO MORE!!!).
Saves with more than 1 neutral killing random become extremely unstable and random due to this and almost impossible to balance. Even though many games town can come out on top of a serial killer + 3 maf game. Look back through your replays and count the number of games the evils (especially the mafia) managed to pull a win when the neutral killing was an arsonist .
(the arsonist gives a lot more time for town to investigate and find mafia because he is not an insta kill - he will kill all the town on n5 AS OPPOSED TO: killing them one by one and reducing the amount of visits / help the town can provide because it dies earlier. The serial killer helps other evils when he kills a town because he stops them protecting / investigating. However the arsonist does not benefit the other evils when he's the last evil alive when there's 4/5 players remaining and he kills 3 or 4 to secure a tie-win.)
But what is so wrong with this? Well, basically the main function of adding a neutral killing into a save is to make life harder for the town alignment specifically. Think about it, 4 maf would make it too hard for town so if a save looks to easy people will say "town has it too easy, add another neutral killing". The neutral evils in general are there to make life harder for the town and in the modern save there is almost always 1 neutral killing. Because arsonists kill much less people at a much slower rate, often whether the neutral killing is a serial killer or an arsonist will break the game.
Balance of individual roles doesn't really matter because that can be fixed in a setup. However when the setup randoms have roles of such varying ability e.g. an arsonist who does far less damage to the town than serial killer.
There have been counter-arguments of people saying there are alternatives to using the neutral killing random. For example using more neutral evils, guaranteeing there is a serial killer / arsonist. However, this doesn't change the fact 90% of saves use the broken random neutral killing. Sitting here and saying "well the save could be fixed if hosts put the effort in" will not make hosts put the effort in! Neutral killings have been commonly used for months now, what makes you think they'll stop now?
So this is why you, developers of mafia, need to do something about the neutral killing random. You ultimately are the only ones who can physically change the neutral killing random to make it balanced or change the neutral killings themselves so you can keep the neutral killing random the same.