Has anyone rented a car from Europcar at Piazzale Roma in Venice?

I did. 10 years ago. Actually I rented in Florence, but they didn't have the car I had booked, so they upgraded me when I drove to Venice.
I rented through AutoEurope.
I don't follow the stars on internet. People who post are generally pissed about something that happened to them. If one had no issues, does not post anything in most cases.

I don’t pay much attention to online reviews for reasons Roberto lists

Someone has to be really ticked off to leave a crappy review and without knowing full circumstances take with a grain of salt

The reviews on booking.com however are trustworthy as only folks who have stayed at property can submit

Reviews on Trip Advisor can be posted by anyone

I can't remember if we rented from Europcar from Venice in 2001 (and then drove to and around Sicily), but almost every year since then we've rented directly from Europcar from various locations around Italy and have been very pleased with them. Really, no problem that I can think of with the people, cars, or locations. But of course a particular location can have a bad manager or bad employee for a time and rack up some bad reviews --- I don't read rental car reviews, but I do sometimes read Italian restaurant reviews, and if some poor reviews are detailed enough and recent and seem reasonable I would weigh them carefully and with a large grain of salt against all the positive reviews.

All car rental searches in and around Venice (Padua, Vicenza) are taking me to Marco Polo Airport for pick up/drop off.

Reviews on booking.com? I stayed in a hotel that was the worst place I ever stayed. I gave an honest review and booking.com spit out a high rating that I couldn't change. I avoid booking.com and hotels.com now because, after a review of good and bad stays, I found my worst stays were booked through one of those two sites.

if you decide to rent on the islands of Venice be aware of the very low speed limit (40 kph if I remember, or 70, could be wrong) on the causeway. Frequent speed cameras cut no slack. And don't drive in the bus lanes, cameras there too.

Renting in Mestre or wherever you are going removes that stress and potential hich costs...