It’s a weak trait. Go over to bloodmallet .com and check the frost sims for Azerites. For outer traits you want 3x Flash Freeze more than anything, then aim for things such as Heart of Darkness, Tradewinds, and Clockheart - as these all provide Mastery (by far your best stat).
Edit: Also wanted to add that you do not have to always rely on Brainfreeze procs to cast Glacial Spike. You want Glacial Spike to hit frozen targets (which the insta-cast Flurry does thanks to Brainfreeze). But for non-bosses you can also just use Frost Nova or your pet’s Freeze ability to manually freeze targets and get off a big Glacial. Not relevant for bosses but very useful for open-world, M+ trash, and some M+ or boss encounters that have adds that can be manually frozen. This is especially good when there are two targets since Freeze and Frost Nova will freeze both targets and allow your Glacial Spike’s cleave to have the increased crit chance as well.