When buying your millwork have a way to check the moisture level of the millwork. Any thing above 15% will shrink after instillation; a level of 7-12 % is better. Prior to installing the millwork sell the backsides of the to keep the moisture from migrating back into it.
Trimming it’s self is fun and challenging there a good trimmers, incredable ones and there are hacks which most of us won’t even let them people frame their so bad.
Trim must be installed super tight fits in all joints corners and seams, remember there is still moisture in the wood still leaving it and as it does so the material will still shrink.
This is why a new house has more millwork cracks at the joints, corners and seams is because the wood in the wall framing is still wet and has not dried out.
When it does the wall thickness will shrink leaving gaps behind the trim and sheetrock surface.
It’s not a bad trim job but instead the contractor was in to much of a hurry to get the house done and did not take the time required to dry the wood framing out before insulating walls.
This is another reason you have sheet rock nails popping in your house. Remember this the dryer the wood the stronger it is.
It holds nails better if it is dry and more weight.