I think we have all been in a situation where we have seen something we liked: on a menu, on an advert, or even on a flash sale/ deal, purchased it, only to be disappointed in the result.

Steak not done to that medium-rare request, a video game controller that started drifting after a few months of use, or a hole in your new clothes that you only notice when you get home

Do you do the British thing of stiff upper lip and complain to your mates about it (or should I say the Karen thing of telling all your friends), or do you negatively review your purchase?

I think it's easy to make complaints via customers services online (albeit time consuming) but if you are willing to keep receipts and chase then usually it results in a resolved case.
In-person though it is a little trickier, especially when they are overly nice about it. I had a Wagamama's waiter ask whether I wanted my ramen re-made three times because I had found it too spicy!