The older stuff (which includes the trooper and snow kits) have been suffering delays. the new stuff has been coming pretty quick.
Their seasons, as far as shipping, literally correspond to the season of the year as in Summer is June 21 - Sep 20 (give or take a day or two) for example. I got an email last week on a FO trooper helmet saying it's still on schedule for Summer 2017 (which has like 11 days left), so we'll see how that turns out.
In the grand scheme, they're at the mercy of their vendors and their orders aren't close to be on the large side so they're more apt to being bumped around i'd imagine. Plus, you're at the mercy of those vender's abilities, and then the QC of LFL and Disney.