Are chrome-cobalt full cast crowns hard for lab techs to work with and polish? Also, can the dentists adjust the margins?
Yes and limited...
With what did you end up with, in the first round?
If you want something durable, easily polished and the ability for the dentist to adjust the margin, go with a full cast gold restoration. As long as the gold content is around 50%, you will be fine.
However, if your first crown was a cast crown, too, I would make sure tho get the same alloy as in the first restoration! Occasionally, different alloys can play tricks in a patients mouth!
And on that 'margin adjust' thing, if your dentist preps and takes an impression, as he should, there is no need to 'adjust' the margin at the time of insertion.
You seem to be concerned about what goes into your mouth, as I would be too. So, make sure your dentist/technician uses quality materials, like a high noble gold alloy. It may cost you more, initially, but you will be served better in the long run.
M