Whenever possible, use the "Preview" feature to look at images or stream a few seconds of video to ensure the content is what it claims to be.
Free users have a limited amount of data they can download every few hours. If you are trying to grab a massive 50GB meganzfolder, you may need to download it in chunks or upgrade to a Pro account.
Because MEGA cannot see your data, it has become a haven for users in regions with strict censorship or for those who simply don't want "Big Tech" scanning their personal files.