Are resistance bands good for building muscle and a good alternative to weights?
They are great for building muscle. I wouldn't say they're a good alternative as they are even better when added to weights.
Using resistance bands with weights is called "accommodating resistance" which means that bands help exercises get harder when they would normally get easier.
For an easy example, think about a bicep curl. By the time you get near the top, the exercise becomes easier. But if you utilize bands, the exercise gets more difficult at the top as the resistance continues to increase.
Louie Simmons, world-renowned strength and conditioning coach and owner of Westside Barbell (one of, if not the, strongest gyms in the world) utilizes bands frequently for accommodating resistance and has stated:
If you want to excel at powerlifting or any sport, then you must develop speed strength, increase acceleration, and gain absolute strength. Bands and chains can be instrumental in developing these aspects of strength. I highly recommend that you try them as soon as possible.
But if you're looking to use just bands, that can certainly be an option for at home workouts. I use them to build the muscles that help maintain posture and to increase blood flow throughout the body and they're a staple for my Esport Specific Training. I discuss how I use them a bit in my post: Exercises for Gamers & Esports-FIX Upper Crossed Syndrome.