It's more true than false. Back in the day, I smoked my share, lived in that scene, and can assure you that if you have potent marijuana, or are inexperienced using it (especially if you consume too much edibles), or have anxiety disorder or other psychological or medical issues, it can sufficiently impair your driving to the point of being unsafe.
This is particularly true if you're also drinking, that you might be under the legal limit for alcohol but combined with the marijuana intoxication, not be safe to drive. Some even argue that if alcohol was a new drug, it would never have been made legal today.
It can lead to harder drugs for two reasons...
1) It removes the blinders for those who thought marijuana was harder than it is, so they wonder about other drugs. Face it, gateway drugs are a real thing by not only showing a person new mental states they never experienced before but also #2 below.
2) The circle of friends and dealer network causes exposure to harder drugs, and for younger people, peer pressure. Several people I smoked pot with back in the day, went on to harder drugs. One ended up so bad that he did a terrible thing and is now serving a life sentence in prison. Could be coincidence that he started out with marijuana, but I was there, in his circle, and saw how it evolved until we lost touch ~20 years ago. It is very unlikely that he would have jumped from no drugs to hard drugs, without marijuana in the middle.
Marijuana increasing other crimes? Yes/no/maybe. If you make the leap that marijuana is a gateway drug, harder drugs do tend to cause criminal activity to generate income to get them. Blame the drugs or blame the druggie circle of friends, it ends up being the same difference.
What marijuana does more than anything else, is wreck a person's ambitions so they lose out on opportunities in life, often at a critical point in their life, like dropping out of or never going to college, then their circle of friends tends to change to be others that do use harder drugs.
It's not just a drug, it's a culture. Legalization can help keep younger adults insulated, one step away from the dealer, harder drug network, but it is still only one step away rather than two or more.
The above does not mean that I am in favor of government regulating what we can and can't do, as long as it only impacts the individual with no harm to others.