It is too low, and there are too many exemptions
“Many Venetians are sceptical of the fee, if not openly hostile to it. The city’s opposition parties tried their best to scuttle the plan; even Italy’s minister of tourism has voiced her disapproval. Critics argue that the admission charge turns Venice into a theme park and does little to address the heavy impact of tourism on city life. Near the station, a few hundred people protested against the initiative, bearing signs with slogans such as “No ticket for Vene-land”.”
Fonte: The Economist