In my experience they're about 75% to 37.5% less polluting. It's a bit of an oversimplification though, as they're 100% less polluting for short trips... so it varies wildly depending on how they're driven.

I got 2l/100km on average when I lived in a flat area close to the city center, and 5l/100km when I had one while living in a suburb 20km away from the city center, vs 8l/100km if I had got the petrol only version of the same car(the car it replaced averaged 10l/100km though, and was 15 years old)

I think the main issue is there's some tax incentives to buy a PHEV in Europe but not as much incentive to actually recharge it at home or at work, which results in a lot of people not making enough use of the battery if they just bought it for tax purposes and don't bother to charge it much.

Also most older model PHEVs have a much too small capacity battery: the one I had, had an 8kwh battery, but if I had a newer one with a ~20kwh my fuel consumption living further from the city would've remained around 2l/100km.

Also with better charging infrastructure, if you can charge at your destination that increases the EV only distance a lot, but there needs to be slow chargers available in basically every carpark and people actually using them to be able to see the benefit them population wide.

TL:DR: skill issue 😅