you can't ask questions[\quote]
That depends on the teacher. Find a teacher who will answer questions about their videos.
Many people who teach live often hide their answers in mysticism or philosophy so finding a teacher who will answer questions is a problem that any Tai Chi may face.
students will get it wrong and think they have it right / no one is there to correct you
This is a common problem with many videos but its a problem with the instruction. Any good video will contain tests, exercises and drills that can be done by yourself or with an untrained partner that will allow someone to see and feel whether they are performing a skill or movement correctly.
This problem exists with live instruction as well. There are plenty of folks out there who have trained in bad habits because their teacher didn't know any better or just didn't bother to correct them.
You can learn from video but only if you already have a lot of previous experience
This is the fault of the instructor for calling something a beginners dvd and then not breaking down the material in a way that beginners can understand.
Internal stuff has to be felt. It can't be seen so it can't be taught on video
This may be the case with a few very high level internal skills, but in most cases Good instruction and tests & exercises like what I described above will overcome this obstacle
All of the criticisms above are things a prospective Tai Chi student should watch out for but they are true about live training just as often as they are about video training.
Finding good instruction is not about the format (private lesson, small class, seminar, video or whatever) It's about finding a good teacher no matter what format they choose to teach in.
Being able to communicate well is an essential skill for anyone who is teaching. Any good teacher is continually learning, improving, refining and honing this skill.
Unfortunately, in the martial arts pretending to teach is so commonplace that many people can't tell the difference. In fact many folks think they're teaching when they are actually pretending to teach because that's the way their teacher pretended to teach them.
Ultimately any failure or breakdown in communication is the responsibility of the one doing the communicating. Blaming the setting, the medium, the format or the student is only an excuse.
Any good teacher will admit this because they are students themselves and they know that recognizing and understanding their mistakes is the only way to improve.