YouTube has helped me significantly, but I must offer a caveat here. YouTube led me to Shawn Clement and Wisdom in Golf and Shawn has helped me understand the golf swing with his method of task-oriented teaching and the specificity with which he describes dynamic motion vs. static positions. I began following Wisdom in Golf in 2007 and it was a total epiphany; I understood things I'd been taught and tips I'd been given for years and years but had never truly registered before. It was a major leap forward in my understanding and aptitude. (...and confidence!)
So, with that stated, YES, YouTube has been a tremendous help to me personally. I would, however, suggest a high degree of discernment to anyone searching for a quick fix or random tips on YouTube. Instructors and influencers are interested in producing content to generate revenue and that may well come at the expense of clarity or adherence to a trusted methodology or philosophy in deference to quantity. YouTube golf instruction has its share of pitfalls to be sure, but there are some real gems to be found out there!