Some advantages of online voice lessons:
1) No travel time, no snow days
2) More options of instructors, and thus:
a. flexibility in when you can take lessons
b. access to more highly skilled instructors than may be available locally
c. possibility of paying less due to more competition
3) If you or your instructor move, you won’t need to find a new instructor; you can also take lessons while on vacation
4) Physically safer (not that voice instructors are typically dangerous, but barring your computer exploding, meeting online is physically safer than in person, such as through less chance of catching airborne infectious diseases, etc.)
Some advantages of in-person voice lessons:
1) Instructor may have a better space for singing full volume without bothering anyone
2) No dependence on internet connection quality or need for a cell phone/computer
3) In-person human connection
Note: good instructors with experience doing online lessons should be able to identify vocal issues with the same or nearly the same efficiency as those doing in-person lessons. Also, online lessons are easy to record video of (with permission), but in-person lessons can be readily recorded (again, with permission) with your phone as well. Ultimately, your choice of online or in-person lessons depends on what your preferences are/what you value, the above are just some things to consider in your decision.