Currently, all you need to know is posted in the stickies in Announcements. Please read the Announcements section because part of its purpose is to keep you safe. Briefly:
1. Some types of metal tube mods with two batteries fitted have a history of explosions and rocket-mode fails.
2. No other type of mod is involved.
3. You might want to avoid what looks like a gas-tight sealed metal tube mod with two batteries unless the manufacturer can prove to you that it cannot explode.
4. A metal tube mod of any kind with large slots or holes distributed along the length of the body cannot explode as it is physically impossible for it to do so. Therefore, it doesn't matter if it runs one or two cells.
5. Don't buy unprotected Li-ion batteries, no matter what anyone anywhere tells you - they cannot possibly be described as safe. They are almost always the culprit in mod explosions. There is a school of thought that they are always the culprit, because other types that seemed to be involved were in fact counterfeits: a safer type of cell was replaced by a counterfeit Li-ion cell with a false label, bought as a result of fraud.