Why are a lot of chunks missing?
uNmINeD can only show chunks that are already saved in the game.
Bedrock Edition saves much less chunks than Java. Increasing the simulation distance and walking around in the game could help.
World pregenerator tools or mods for Java Edition (Chunky) can also be used to fill the missing areas.
Is it safe to use uNmINeD while Minecraft is running?
Java Edition: most likely yes, as no one has ever reported a problem in the ten-year history of uNmINeD.
Bedrock Edition: maybe yes, but uNmINeD is likely to crash when world files change.
It’s always a good idea to close Minecraft before using an external tool, and make regular backups of your worlds to prevent data loss.
Is uNmINeD a malware/virus?
No. Some cautious antivirus tools may consider it “dangerous” because it is a less common software that relatively few people download, and it is not signed by a certificate because software signing certificates are expensive.
If your antivirus software says it is a malware, send them the downloaded package for analysis, so they can improve their software to have fever false positives.
Be careful not to download uNmINeD from anywhere other than this site (unmined.net). There are plenty of malicious sites that offer fake minecraft tools for download.