Another difference is that the layers in shallots are much thinner than in onions. So, they can easily be cut into smaller pieces than onions, since all of the onion-things, tend to fall apart at the layers, when you have cut them up.
I find shallots to be a little stronger than many (but not all!) onions, and I often use them in things that call for a small onion or small amount of onion, since I rarely have small onions on hand. I use a small shallot when making quacamole, and I rinse it after mincing it. And the things I use the most in are Thai foods, which got me started growing them! A lot of southern Indian foods use shallots, too, partly because of the strong flavor, and maybe because they grow better in that area, as in Thailand.
I have an idea about why shallots are often called a "cross" between onions and garlic - while they don't really have a taste of garlic, they sort of grow more like garlic, in sections, and often when the shallots are peeled, there is a smaller and larger part, even though it looks like an elongated onion at first. And, like most hardneck garlic, it can get planted in fall, and grows through the winter, and takes off in the spring. One shallot "set" can produce 6-8 shallots in a cluster, once harvested.