I'm no expert but when I rebuilt my wire harness I did my research and lots of joker's talking about not extending wires but never gave intelligent reasons. My family is full of electricians and they and other research I did lead me to believe that as long as you use the proper methods to joining 2 wires together and use the proper tools/material there is no issue extending wires that will perform well and last a long time. Cheap wire is a bad idea, and cheap material to connect also bad idea. Anything I extend I get good solder and make sure I don't use too much as that can cause problems too. Shrink tube, proper heat, Etc. Etc.
I've done a couple harnesses over the years and never had any problems. Dyno tuners never noticed, and those vehicles are still driving around, one of them is a weekend racer
But again I'm no expert and won't claim my comments are legit but in my experience your fine. There is a distance variable, but inside an engine bay your normally not gonna reach that limit..