Tango learning falls on a spectrum. Anecdotally (only!):
Some dancers I know have never taken tango lessons – group or private – except perhaps the odd open, drop-in, or pre-milonga class. They learned from a hodgepodge of YouTube videos, home practice, and above all tips begged from experienced dancers at prácticas and milongas. Most of those who ended up able to dance tango already knew folkloric, Latin, swing, or other dance. By virtue of having no contacts from classes, those who survive are perforce hyper-social and thus popular despite their shortcomings.
Many friends started and ended their tango learning in group classes. They found the most affordable school, went through its level system, and at the end of that decided that they didn't want or need more lessons. From there they proceeded much like the previous group. They're often worse than the previous group, though – they're at the peak of "Mount Stupid" on the Dunning-Kruger curve. Still, they have social contacts from their classes, so they dance a lot.
Then there's me. I started tango in early 2013 and took my first private lesson before the end of the year. Since 2016, I've taken weekly private lessons exclusively. Group classes served three purposes: (1) to make social contacts at or around my level at the start, (2) to give me ideas for my private lessons, and (3) to let me serve as an accompanier on the way to possibly learning to teach. To be honest, when I left group classes I was in the "Valley of Despair" on the curve; private lessons are helping me climb out. This approach isn't great socially . . . I've been called everything from "anonymous" to "snob," and only other technical tango addicts can see the work I do in my lessons.
Finally, there's my friend who's an overachiever with money. He dislikes waiting for anything or anyone. He regularly up and decides to fly planes, become a karate black belt, ride horses, scuba dive, or whatever. He finds the best local teacher he can, pays whatever it costs, and goes as often as it takes to become proficient as close to overnight as possible. When he decided tango interested him, he skipped directly to the founder of a respected school and prepaid a year of private lessons with her. He does everything by the book but with zero creativity . . . 8/10 for technical merit but 2/10 for artistic impression. However, his money also bought him introductions to the right dancers.