Probably, APFS is more reliable than Mac OS Extended because it uses checksums on metadata to prevent your data from getting corrupted. As for Mac OS Extended, it provides no protection against file corruption at all.
Compare to Mac OS Extended (aka HFS Plus, or HFS+), APFS (Apple File System) is a way more modern file system. It made its way into the Mac in 2017 with the release of macOS High Sierra, and replaced HFS+ as the new default file system for Mac drives. Also, it is designed mainly for SSD and flash storage devices.
In addition, since APFS operates on the space-sharing feature and does not require manual storage space partitioning, you won't have to worry about your files becoming corrupted. It offers dynamic storage distribution, which intelligently distributes space to each partition based on how much space they require.
Now let's talk about its competitor, Mac OS Extended. You can take it as the modified version of HFS. Unlike APFS, Mac OS Extended is a better choice for mechanical and hybrid drives, and it is the default file system for Mac disks since 1998 until its younger brother APFS replaced it in 2017. Also, an HFS+ formatted drive can be used by any version of Mac, making it more compatible than APFS since the latter can only get full read-write access on macOS High Sierra and later versions.
To wrap up, APFS would not tend to corrupt your files vs Mac OS Extended unless some unexpected situations happen when you clone the User folder to your external SDD, like a sudden power outage.