I'm curious. Why do people think the Soul Grinder has improved all that much this edition? Is it just the hull points thing?
Well there is a few reasons why they have gotten better in 6th Edition.
For starters though, it is worth noting that they were never really uncompetitive, it was just that they suffered because they were the sole vehicles in the codex, so as a result, all the enemies Anti-Mech weapons (which were always brought because 5th was mech heavy) concentrated their fire on them. However
1. There are now some more vehicles in the army (Fleshshredders/Chariots). They aren't great, but at least its something else for the enemies anti-mech to worry about and hopefully target (hopefully!)
2. 3 Soul Grinders with Phlegm is nearly 100pts less than two of the competitive build Daemon Princes.
3. They are impervious to small arms fire, Dp's can and will get torrented to death.
4. Pie-plates now effect vehicles even if they are only partially under it (in other words you don't need a direct hit anymore.
5. There is no wrecked roll anymore so they are statistically harder to destroy unless shot by weapons that are Ap1. In other words, they completely ignore half the vehicle damage chart.
6. Weapon destroyed rolls are now random so you no longer instantly loss your vital mawcannon.
7. If they are 25% covered they get a cover save.
Those are a few reasons why Soul Grinders are much better in 6th Edition.