Powder compacting when seating bullets.

You compact it enough and it won’t compact anymore and you can’t hit your proper lengths. Actually start crushing powder and you make more sirface area which makes it burn faster.
Also, sometimes it can push the bullet back out a bit depending on your neck tension.
Nothing inherently bad by compaction on its own.

Compressing a load has always made my seating depth vary by a few additional thousandths. It can also lead to a more pronounced ring around the bullet from the seating stem (which rarely affects accuracy as much as you think it would). Also, as stated above sometimes the bullet can actually back out a little if your interference fit isn't tight enough.

Compressing a load though isn't wrong by itself, depending on powder burn rate and corresponding pressure. Drop tubes like you mentioned do make it easier to get more powder in the case and often compact it more uniformly (less variation in seating depth).

I try to have just the tiniest bit of compression if it is easy to get. It helps keep the powder column always consistent in my opinion.

I think I remember reading that very mild powder compression (say 103% capacity) or so was beneficial as it resulted in more uniform powder packing because powder would not shift around like it would in a case that was partially filled. I have no data to back this up.