It's mostly about the dose of carcinogens. THC is far less addictive than nicotine, so on average people who exclusively use cannabis inhale less smoke than people who smoke tobacco. A pack a day of cigarettes is a very common amount for people who smoke tobacco. That's 20 cigarettes, or 14-16 grams of tobacco a day. Few people who exclusively use cannabis are smoking 14grams/half oz a day.

There are two other major factors limiting our knowledge of the health impacts of cannabis smoking: 1) it remains federally illegal and therefore hard to get research funding. 2) a large proportion of the studies over the past several decades with people who smoked cannabis were confounded by many of the participants also smoking tobacco

TLDR: it's not great to inhale smoke of any kind, as it does contain an abundance of carcinogens.

There's a growing body of evidence that pot smoke can cause/exacerbate emphysema. It's been nigh on impossible to study the actual health risks associated with marijuana smoke due to it's legal status in most countries. Now that legalization is taking root, don't be surprised if we find out over the next 10-20 years that smoking pot is just as bad, if not worse than tobacco.

The differences in the way people smoke pot over tobacco (unfiltered, inhaling deeply and holding it in vs filtered, short, quick breaths) is known to deposit more tar in the lower respiratory tract. So even if there are fewer bad things in pot smoke per given volume, a lot more smoke is sticking to your lungs, increasing exposure and causing damage.

If you (or anyone else for that matter) are set on inhaling anything other than air, it's best to vape from a harm reduction standpoint. There's several orders of magnitude less nasty shit in vapor than smoke, no matter the substance