Warlord, Mantic and GW all seem to have similarly good customer service.
Typically it helps to go through the seller. For later issues you can do that too, but mentioning prior interaction seems to be enough, at least with Mantic.
Usually they ask for photos of the issue.
This is great customer service, however it is ripe for abuse.
Eaglemoss, and probably most partworks work the same, in that case I didn't even need to send a photo of magazine damage to receive a replacement, which was also damaged, so they sent a replacement, so on. How many Klingon destroyers does one need? They never learnt, so I ended up cancelling.
I would assume people are already playing the system.
I've been thinking of it, in a good way, buying for the risk and notify companies of any fault and accruing replacements.
But I'm sure there are people just saying things are broken or purposely breaking things, only to reglue and resell.
I suspect numbers of complaints are few though, in comparison to total sales.