So some people are saying they do, others are saying they don't. For these purposes lets just say they are playing an un comped game of battleline against a another top competitive payer.
Usually it would make sense to write the best list logically possible and naturally this may include a death star, so against an average or below opponent who doesn't know how to deal with it, or feed it chaff. It seems as though the odds will be in your favour before you've even started.

But against another competitive player who knows how to have it chasing chaff perfectly placed around the entire board and hitting nothing of value the entire game- wouldn't it seem rather pointless to put all "your eggs in one basket". Not to mention if it gets wiped out you have very little amount of points in other units who can actually do damage for you.
Then wouldn't this be a different matter if you ran the 'Two Towers" list of a horde of executioners and Witch Elf/Cob. Because now the experienced player has two units that could potentially do so much damage.

This brings me onto the multiple Master and BsB on Dark Steeds with Warlocks build. Wouldn't this be the most competitive /viable option you could take no matter how well you can use it? M9 fast calvary. 4++ to the Warlocks. While the masters sport a 1+ in combat. While other Dark Riders wipe off enemy chaff. Your deathstar unit is vanguarding 12, marching 18. At least guaranteed a charge against something worthwhile in turn 2. Making all the anti-deathstar tactics kinda pointless.
Can someone aware me, this is the extent of my knowledge on this so I don't really know how it all works in reality.

The bro-star is very nasty. It is also very expensive like most deathstars.

For some armies (Ogres for instance) the only competitive builds are deathstars. So that is what you see with them. But for other armies, like DE or Empire or chaos ... more thought has gone into the army books and there are multiple approaches to success. Which makes them more dynamic to collect and play.

So sometimes the sword of anti-heros is super awesome, and other times it is very average.

Scout unit 12" in front of bro-lock death star so they can't vanguard. Depending on the army, another scout unit behind a vanguard a unit up behind it for double flee.

Also, feeding a death star chaff isn't the only option. You can pin deathstars and set up flank charges with some heavy hitters and wipe the deaths at out.

Warlocks are also just toughness 3 so the can take loses from missile shooting. 4+ ward is good but its still just 50/50.

My empire list could throw a steam tank into it. Hit the flanks or even rear with demigryphs or a knight unit with characters.

This why we pkay, because its not a done deal in the list building phase.

Can someone explain to me what the bro-lock deathstar exactly is ?

Simple, 10 or more Warlocks with 5 combat characters in the front rank and a sorceress in the second rank. Super Expensive, Super Destructive on the charge and still has great spells to use.

Yes they do, but its there is a high risk that if your opponent has a counter, you lose all your combat power, and most of your army, in one go. And to do good you need to constantly be able to win, a death star has an a huge weakness, and that all the points in one place. Those type of list seldom do well consistently, once in a while why not, continually not possible.

Deathstars are ultimately a tactic of weakness, not strength. You have NO fallback position when that unit dies, or gets maneuvered out of position, or just plain tarpitted with something cheap.

Sure, it may be HARD to kill that unit, but when it does you're screwed. It may be hard to bait a fast-cav unit out of position, but when it does happen you just lost the game. You may not HAVE to take that bait or charge that tarpit, but a skilled player will leave you few options.

In short, do it if you want to... but expect the GOOD tournament players, who have been dealing with deathstars from day one, to had you your head.