As the title says, I believe that the Sudden Death 'feature' coming in patch 3.55 is going to be a poor one for a variety of reasons, which I will now explain.
We got very little information on this change, however, I think it is safe to speculate that "Sudden Death" will simply be a quickly stacking Culling Time which happens at the 8 minute mark if both teams are tied. Based off this hypothesis, my following points are made.
1) This system benefits specific jobs:
Simply put, some jobs benefit more from this change than others. Jobs like Paladin, White Mage, and Summoner have huge cooldowns to take advantage of a Sudden Death mechanic at this point in the match. Abilities like Sacred Prism and Hallowed Ground, for example, will now become more valued than ever. I have no suggestion here, only that there will be a further imbalance of jobs in the event of Sudden Death.
2) Melee Limit Break is now even more broken:
As if losing in the last 30 seconds due to a melee limit break wasn't cheesy enough, it will now be even harder to deal with, bordering on 'impossible' depending on the damage amplification of Sudden Death. At 2 stacks of culling, melee LB can one shot anything but a tank outright, without any need of assistance from teammates. There are very very few counters to melee LB in this condition, and even those counters are hyper-conditional as well, such as a healer having both purify and attunement ready (and have it register in time), having tank LB, etc. Even the typical counters like Testudo, Thrill, and a well timed benediction will not save you here.
3) Strategies become more 'cheap':
Now, instead of trying to close out a draw and turn it into a win in the last minute of the match, teams are more likely to "wait it out" until Sudden Death. What will happen is one team being hit by literally unhealable damage due to Sudden Death's damage amplification and losing in a total cheese. It's fair to argue that this is legitimate, but my opinion is that it takes away from the defensive options available, making the game far less about skill and more about which job has large cool downs come up at this time.
My hopes are that SE actually considers player feedback on this change during preseason (unlike several other changes which were correctly predicted to be detrimental to the nature of The Feast), and takes action accordingly.