Mandos is the great judge of the Valar, and his decisions are widely respected by all the other Valar. But most of his judgements have disastrous consequences for the Valar, the Elves, and all of Arda. If anyone could have seen the consequences beforehand, it would be Mandos, who is granted greater foresight than any of the Valar.
When some of the Valar worry about the imminent awakening of the Elves, and the possibility that Melkor will find them before the Valar do, Mandos basically tells them not to worry, because the Elves won't wake up anytime soon. The Valar accept his judgment, and abandon their plans to capture Melkor before the Elves wake up. The Elves awaken shortly after the debate, Melkor finds them, he abducts and corrupts many of them, and misleads many more, before Oromë stumbles upon the remainder.
When the Valar debate over whether they should bring the Elves to Valinor, the crucial vote comes from Mandos, who says that the Elves should indeed come to Valinor. This, too, proves disastrous. The immediate consequence is the sundering of the Elves, but the long term effects will be far more sinister.
When Melkor is brought to face judgement for his crimes, Mandos agrees with the inexplicable decision to allow him to appeal after a relatively short time in prison in the Halls of Mandos. Melkor serves his sentence, then pretends to be remorseful, and is freed. We read nothing to indicate that Mandos expressed any concerns about this course of action, despite the fact that Melkor is the most evil and powerful being in existence.
When Fëanor threatens his half brother, Mandos exiles him from the city of the Elves in Valinor. This sets the stage for the next step in the sundering. Fëanor begins to believe the lies told by Melkor, that the Valar are holding the Elves captive, desiring to steal Fëanor's beloved Silmarils, and keeping the Elves out of Middle-earth in order to allow the still-slumbering Men to take control of the continent in the Elves' stead. Fëanor and Finwë, as well as Fëanor's sons, build a fortress and store the Silmarils inside.
When Melkor and Ungoliant kill the Trees of Valinor, and the Valar ask Fëanor to give them the Silmarils so the trees can be healed, he refuses, saying he loves them so much that losing them would break his heart, and he would become the first person to die in Valinor. Mandos, rather obliquely, says "Not the first". What he (and he alone) knows, but does not say, is that – even as they speak - Melkor and Ungoliant are murdering Finwë and stealing the Silmarils. Had he spoken sooner, or less cryptically, this might have been averted. But he didn't, so Finwë dies and the Silmarils are carried off, with catastrophic implications for everyone.
After this, Fëanor is so infuriated that he leads the greater part of the Noldor on a fool's errand, beginning a vendetta that will result in many deaths and untold suffering. Fëanor and his followers take an oath to kill anyone – even the Valar themselves - who stands between the Noldor and the Silmarils. The Teleri refuse to help the Noldor (who are their kin) and are slaughtered by Fëanor's followers as a result. Fëanor and his allies pursue Melkor out of Aman and into Middle-earth.
- Mandos then appears before them and pronounces "The Doom of the Noldor", also known as "The Curse of Mandos". He tells the renegades that their vendetta will lead them to ruin, and informs Fëanor that he is banished from Valinor forever. The only person who could have convinced the Noldor to repent was Fëanor himself, but having been banished, he no longer has any reason to reconsider his present course of action, let alone attempt to persuade his followers to turn back. In time, this will lead to the destruction of much of Middle-earth, many, many deaths, and all manner of horrible events over the course of the next several thousand years.
Why are Mandos' judgements so widely respected when they often prove to have monstrous consequences for all of Arda? On a related note, is he simply predicting the future, or is he deciding what will happen, in accordance to the will of Eru? Did any of his judgements work out well, or were they all so catastrophic?
Note: While Tolkien sometimes uses the word "Doom" (often in relation to Mandos, the "Doomsman of the Valar") in the sense in which we think of it – i.e., a terrible fate – he more often uses it in the sense in which it was originally intended in Middle English – meaning simply "a fate or judgement". Sometimes, when used in this way, a "doom" can be a good thing.