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.