What are repacks
Repacks are pre-compiled versions of common WoW emulators, such as vMaNGOS, AzerothCore, TrinityCore, etc. Being pre-compiled means you don’t have to compile them by yourself skipping quite some work that you and your PC should otherwise do when installing such emulators following the standard installation procedure.
So a repack is basically the binary files that you can typically just download and execute. For example mangosd.exe and realmd.exe for MaNGOS-based emulators or worldserver.exe and authserver.exe for AzerothCore/TrinityCore-based emulators).
Sometimes those pre-compiled repacks are provided by the official project contributors. For example, vMaNGOS repacks are being officially provided by brotalnia, the maintainer of the project. In cases like this, there is absolutely nothing wrong or risky in using them.
Usually, however, repacks are created by other people around the web and they distribute them without the source code. In this case, it is strongly advised to avoid using such repacks.
Why (unofficial) repacks are bad
Using unofficial repacks (i.e. binaries provided by users who are not official contributors of wow emulation projects) is both risky and inconvenient. There are actually several reasons you should avoid them.
Security. You never know what is really running on your computer. Repacks might contain malicious code that can execute undesired operations inside your system. Including keyloggers that can steal all your passwords. Some WoW server repacks have been proven to run bitcoin mining, exploiting the CPU of users downloading them.
Control. You cannot modify the source code, because you do not have access to it. Yes, you can still update the database and do many customizations, but still, you cannot change the core of your server and are limited to the functionality offered by that repack. You cannot apply any core patch or add core modules.
Support. You cannot install updates by yourself, because you can rely only on the author of the repack to publish updates. This can be an issue even if the author does not have bad intentions, but simply stops publishing updates for any reason. No one can pick up his repack and keep publishing updates unless the author of the repack has also published the repack source code. If this happens, you can no longer benefit from core updates, so you can’t get core bugs fixes. Some people keep using old versions of repacks, and they most likely have known security vulnerabilities that have been fixed by the main emulators but not in the repacks based on older versions.
Platform. Sometimes repacks are bound to a specific platform and you cannot easily run them in a different type of environment than the one where they have been compiled for. Think of people running Windows 32 bit repacks on Linux using wine. What a waste of computational power!
Legality. All repacks are technically illegal (unless their source code is provided). WoW emulators are usually released under the GNU GPL license (or any other license that is compatible with it). Such a license requires any redistribution of the software to also include the original source code. When a repack is published without the source code included or being publicly available, it is a clear violation of the license and thus illegal.
What you can do instead
The best thing you can do is installing a WoW emulator by following its official installation guide. Active WoW emulation projects typically have plenty of tutorials explaining how to do it and a friendly community around that is able to help you.
If you are still looking for an easier solution, you can consider the repacks provided by the official emulation community, such as the vMaNGOS one mentioned above, or try other official pre-compiled solutions such as the AzerothCore Docker images.
Nevertheless, try to avoid compiling no-longer-maintained sources as well because they could also have some of the potential issues mentioned above (especially security ones).