Trams have the same problem trains have. If something happens on a tram line (and these are a lot more integrated with roads than train lines, so things do happen), a big segment of the network comes to a standstill. They're not like buses or cars that can drive around a major accident in an emergency, even if that meanns they'll skip a stop or two.

My experience of public transport modes in various cities is at odds with this.

Trams and trains generally offer far more reliable schedules, frequencies and journey times than busses because they either have completely dedicated alignments or have priority where there is any interface with normal traffic.

Most buses inevitably bunch (see https://setosa.io/bus/ for a nice simulation) and/or get stuck in traffic as a matter of routine. The inconvenience may be less per delay but busses are delayed far more frequently than trams and trains on most of the public transport systems I've used. So for regular users, the cumulative inconvenience is much worse on busses than on trains/trams. Which is why people flock to trains and trams when available as an alternative to busses.

Specifically with regard to the parent, the frequency at which unplanned outages happens with tram services in Zurich is extremely low in my experience - certainly planned changes to schedules or routes (for maintenance, upgrades, etc.) are far more frequent. And when "something happens" (i.e. a traffic accident), the path for trams is cleared as quickly as possible - often in 30 minutes or less - so you'd really have to be unlucky to be inconvenienced by such an occurrence.

In most situations you want trams to have own lanes and semaphore priority which reduces collision chances to a minimum. Worst case you can have some spare busses to provide temporary replacement services for such situations or you can divert some buses from other lines to provide services in problematic sector till situation isn't resolved.