Sadly yes. Some bus companies are very reliable (traffic jams permitting), but there are definitely times when they aren't, and I've used rural routes where the hourly bus might just not turn up, which makes them completely infeasible for commuting or appointments.

To replace cars with buses for trips you really need them to be rock solid.

It depends where you are, some areas have very good bus services and other terrible ones. Almost everywhere has been forbidden from having a more centralised approach and so most places are a mash of random companies with very different approaches and reliabilities.