Have rented in Portugal multiple times - once on Sao Miguel
Rented through a large agency on Sao Miguel and in other parts of Portugal. We have used Kemwel/AutoEurope with no issues and they have great 24/7 support. Booking directly, we have used EuropCar and Guerin (booked directly and through Enterprise (specials on their website). Guerin is a franchise of Enterprise. We usually avoid other agencies like booking.com or Expedia as it seems there will be minimal support and occasionally they run out of cars.
Zero deductible does not mean zero deposit. You still have to put a deposit down on the car with a hold on a credit card even if a pre-paid rental just like you do in the US.
We have had cars in Europe hit and break down on us, so we get usually get once of the higher tiers of Zero deductible insurance. With that, we haven't had to pay anything additional. Just drop off the car and go.
It's just not worth it to us to have to deal with any issue with a car. We had cars hit in Italy and Greece - didn't have to do a thing, just drop off the keys. I'm sure if the damage was worse, we would have needed a police report. In Porto, the car started to smoke. Called the rental agency. They sent a tow truck and a pre-paid taxi to get us to the airport to pick up another car. There was no arguing whether there was a problem with the car when we picked it up or we did something to it.
Not all zero deductible coverage is the same. You have to look at what is included as they seem to have different options in the past couple of years. I also was told refundable deductible was not necessarily zero deductible. Some just cover CDW and theft. Others include glass, tires/wheels, keys, lockout, roadside assistance, etc... Some cover your own personal property and injury. The included liability is just for other cars, property and people outside your vehicle. We have paid anywhere from $15-$30 per day for this coverage. It's higher on cars like Volvo, Mercedes, BMW. If you have no insurance other than the liability included and rely on a credit card, check what they cover. Even primary coverage does often not include diminished value which could be added if repairs are significant. Other coverages have varying deductibles. Also, some of the Visa coverages (depending on the level of card) are secondary or have limits. Sometimes they exclude certain countries but don't remember Portugal on the exclusion list.