They're best for things that have sparse and spammy reviews elsewhere (mattresses, random accessories etc.) and bad for any area where you are a power user (computers, specific hobbies). I think of them as the 80:20, not the optimal.

I've loved Wirecutter's reviews for many years, but have occasionally been disappointed by their recommendations. It's hard to say whether their quality has actually gone down or if the success/miss ratio is constant.

Examples:

External hard drive (pre-2016): no mention whatsoever of RPM, which is widely regarded as critical information when choosing a non-SSD hard drive. They didn't even include a blurb saying that RPM doesn't matter.

Bath towels (2019): Pasting my own comment: "I ordered a set of Frontgate towels based on this recommendation. After fewer than five washes (cold normal cycle with standard detergent, line dry), the towels aren't plush at all. The towels arrived feeling luxurious and spa-like, but after only a few uses they feel drab and normal." They updated their towel recommendations a week ago and Frontgate remains their top choice. I feel like I wasted several hundred dollars on an extensive set of pretty average towels. If I'm washing them wrong, it would have been nice for them to reply and update their care instructions.

Broom (2020): It's...fine. Maybe my expectations were set too high, but I thought it would be the Best Broom, not just a broom.

I had a series of very good experiences with wirecutter from 2014-2017, where I bought their bike recommendation, their laptop recommendation, their desk recommendation and their headphone recommendations, all of which were really exceptionally excellent and made my life a lot better. 

However in the last two years, basically everything I bought from wirecutter has turned out to be pretty mediocre. Their headphone recommendations are kinda OK but not great, their towel recommendation is pretty mediocre and lost tons of fabric for the first three times when I washed them (causing all my clothing that I washed them with to have lots of small towel-fibers all over them), their computer screen recommendations weren't that good. Their product categories in a bunch of places no longer make sense to me, and something about the review practices has felt worse, but it is totally plausible that I am just imagining things.

FWIW, I was disappointed in my purchases from the wirecutter pre-2016 (I think back in 2013 or so). I can only remember one distinct purchase (a set of headphones). I'm not sure if I made more than one purchase or not, but remember a cached belief from >2 years ago that the recommendations were underwhelming.

That said, I did use them for purchasing my latest mattress (within the last year), which I am in fact happy with.

I concur with romeostevensit's answer that they are (and always have been) good – I'd say more than good-enough – for things for which you're not a 'power user'. But I've been happy with their picks even for some of those things for which I am a power user.

I haven't noticed their quality declining since their acquisition by the NYT tho I find it plausible that that might happen.

A glaring omission in Wirecutter's laptop recommendations: they say the HP Envy x360 13 is superior to their top picks, but don't list it. This is ostensibly due to stock shortages, but a slightly different configuration (with the 4700U) has been in stock for a while. This is important because it's $300-400 cheaper than their top picks. 

The 13-inch HP Envy x360 with an AMD Ryzen 5-4500U processor is an excellent ultrabook—it’s compact, light, and had nearly 12 hours of battery life in our tests. It has a great keyboard, a responsive trackpad, and a reliable fingerprint reader, and build quality just as good as our other picks. But the Envy x360 is out of stock everywhere, so we can’t recommend it.

They didn't recommend other laptops with AMD Ryzen 4000 CPUs either, which other reviewers say are significantly better and slightly cheaper than Intel 10th generation CPUs. They also didn't test the LG Gram (with an Intel CPU), whose previous iterations were excellent.