Err, they are very easy to debug. You can have them print out every line as they execute and follow along what was done. Prior to systemd, if there was ever anything critically broken, I could step through the entire startup from the initramfs and bring up the system line-by-line or script by script as I needed, and see exactly what was at fault.

With a script-based startup, the entire system was understandable by anyone with some rudimentary knowledge. It wasn't complicated, and it worked well. It did have its shortcomings, but the accessibility to modification and the ability to effectively debug were not amongst them. In comparison, systemd's behaviour is a black box with the unit files as input. The actual internal state is opaque, and it's inaccessible even to experts.