It all depends, really. As other people have pointed out, you can allay the problems of car traffic by giving trams right-of-way, dedicated lanes or both.

I've tried to summarise what I understand are the key pros and cons of trams vs. buses below. As you can see, I've come up with more positives than negatives, but it really does depend on the particular situation in each town or city. Mixed modes are always best, IMO!

Upsides

• Trams are generally easier to electrify than buses, so they can be more eco-friendly in that sense, though this is changing with better charging infrastructure, hydrogen fuel cells, etc., for buses.

• Trams can also be built with very low clearance, making it near impossible for them to run someone over, which is a good safety feature.

• They're generally quieter than buses, too.

Mixed

• Trams cost more to build compared with buses (because you need to lay tracks) but then cost less in the longer term (because rails and metal wheels are more efficient than tarmac roads and rubber wheels, and wear out more slowly). So, which is best from a cost POV depends on your exact situation.

• Tram rails can be laid with grass underneath, which is more eco-friendly for numerous reasons. Of course, this also entails a downside if you want to use that space for other vehicle types. Again, another one where a planner would need to weigh costs and benefits.

Downsides

• The key downside is that they need tracks: this makes them much harder to divert around some kinds of temporary obstructions which buses can easily manoeuvre around.