They have the mechanical complexity of both, in a package of roughly the same size. And usually far smaller batteries and thus far less battery-only range than a pure EV. There are plenty of downsides.
I'm not sure in which "best of both worlds" a standard hybrid is better than a PHEV - a PHEV allows for cheaper fuel (grid electricity) when it's available. That being said, the extra cost is associated with larger batteries than standard hybrids. As batteries come down in price / size, I'm not sure why people would want a standard hybrid over a PHEV.
All they do is slightly improve fuel efficiency; but they cost more to buy, and cost more to maintain. I had to dump my first hybrid because I couldn't find anyone to fix it.
The way to think of it is that a hybrid (both traditional and PHEV,) has more parts than an ICE car, which has more parts than an EV. It's more things that can fail as the car gets old, and more things to pay for when the car is new.
Edit: I should add that hybrids were good for automakers to dip their toe in the water for electrification: IE, get the supply chain working and get institutional knowledge. But, that ship sailed 10 years ago.