Truth be told, it IS a difficult checkride, probably the most difficult after Private Pilot for those that choose to go that path. Not difficult if you are prepared, but you have to be an expert on the standards for private pilot and commercial pilot and able to teach. Not just tell you something, but actually teach you something. Teaching skills take time to develop, a lot longer than flying skills, but you don't get all that much experience teaching before the checkride, you gain that mostly after the checkride, so that makes it more difficult. Then there is safety of flight, that has to be the ultimate thing on the mind of the prospective CFI, that you can always go out the next day or later and do the flight over again with a student, but you let them go too far in a maneuver and you may end up in an accident or worse.

Some of the things I've seen in part 61 were not knowing or understanding endorsements and requirements for practical tests, not coming with the proper endorsements themselves and then not knowing which ones to give or where to find the information. Again, you have to be an expert on the subject and able to find the information and come up with the right answer. Not that you'll have every answer, but you should know where to get it.

Also, using the appropriate references with the student, it should never be a "my CFI told me so", it should be where the CFI showed you where that regulation, practice, maneuver, recommendation or piece of knowledge was located. Get into the CFRs, get into the ACs, get into the standards, the POH, the handbooks, all of that stuff. Your student should trust you because you show them where these things are, not because there's some assumed halo around your head because you have a CFI certificate. Then when the student gets asked a question they don't know off the top of their head, they will be able to go directly to the right place to find the answer, rather than trying to rely on rote knowledge and ridiculous mnemonics or memorized lists of things. Memory aids are great, but not at the expense of the underlying knowledge. We tend to assume everything our previous CFIs taught us are correct and rarely question, but sometimes when you look in deeper you find that what was told to you had no basis in reality. These are some of the skills that make a good CFI.

One of the reasons that inspectors started doing initial CFI checks was what you mention, the abnormally high rate of failure associated with DPE CFI checks. Part of it was due to poor instruction and preparation, but there were other factors that contributed to it. Heck, think about what kind of damage a "first time failure" would cause with the CFI who now thinks it's "ok" to arbitrarily fail someone the first time something is done successful. That's against so many principles of instruction and teaching that it's ridiculous. Unfortunately a few bad seeds always exist, but the better prepared you are and the better you know the material, the better you can stand up for yourself.