The quick answer is that hiking solo is more hazardous than hiking with a pardner or a group, but that is not always true.

How capable is your pardner? Can your bud render competent first aid or hike out, if necessary (note your buddy is now hiking solo and you now are on your own).

It is not unknown in a group for an individual to go along, not paying much attention to circumstances, and then become separated from those who were. This person is now SOL. It well might have been better for the individual to have hiked alone. They might then have paid attention to the route and the situation.

Many people do go out solo and they do not necessarily die in great numbers. One tends to be more cautious and careful. It is necessary to leave info with a reliable person, stick to your planned route, and agree on a time to press the panic button. PLB technology does improve the situation to some extent.

Who is better prepared - the soloist with a FAK, together with training and experience, or a group of any size with neither?

It all comes down to skills and capabilities. Some groups are clueless, and can be more poorly prepared than an individual hiker.

Good topic!