There are legitimate cons to fare-free public transport:

• In places with large populations where fares currently make up a large portion of revenue, removing fares reduces the total funding and may necessitate cutting routes or frequency
• Many more new riders, in a system that is not receiving the automatic boost in funding that would come with fares, may result in being unable to expand to accommodate the new riders adequately
• The extra funding from tax revenue would be more efficiently spent on expanding the network (routes or frequencies) rather than further subsidising the existing routes
• Increases in “problem riders” (though the video addresses why this is overstated)

But there are many more important pros:

• Equity: it may have little impact on commutes to work, but opens up the ability for people to travel to other destinations much more easily
• Lower admin costs: no need to pay fare enforcers, no need to pay for expensive tech solutions for payment systems, etc.
• Operator safety: some studies suggest the main source of conflict between operators and passengers is at the fare box.
• Increases ridership. Duh.

The video makes a bunch more points than just this. The above is only a summary (and already quite a lossy summary) of roughly the 1st half of the video.