I have a repairman cert. on my two E-AB's. Building a airplane is a wonderful experience and you learn much. Indeed you learn enough building the first that the 2nd isn't too bad. I do not have to depend on anyone else to "legally" keep my airplanes in the air. That's great.
BUT
Does that mean that I can safely keep my airplanes flying? Maybe, maybe not. Frankly I need help, sometimes a lot of help. One does not need to know very much about engines and maybe other systems to complete a E-AB. You can buy a new engine or buy a core(or something in between) and install it well enough to pass inspection but you may not really know how to maintain it?
My point is that depending on your experience and ability it will still go a long way to be friends with a A&P/IA who knows your type of aircraft and be willing to even pay for his advice if necessary. YMMV
Over time one will learn the necessary skills but as other certified mechanics have already said, it's most reassuring to at least have another set of skilled eyes look over your work.
IMO there is no easy way to learn to "safely" maintain an airplane. But as long as you love airplanes at least it will be fun to learn. Personally I love airplanes and like working on them more than flying, but some days flying does confirm how wonderful they are. After almost 50 years i still look down and feel sorry for those poor folks tied to the ground.....
With due respect,
Jack