I'm myself still a beginner when it comes to profile design, but I have some strong indications so far that it ain't easy
The first part where you design from test targets or color filter responses is fairly mechanical and automatic, but then to make the subjective adjustments to make it work well with a tone curve and various subjects is the hard part. If you do it manually this means that you need to look at a lot of test pictures and hand-tune manually, test the profile for some time, do some further refinements. It can be weeks before you're satisfied.
Of what I've heard from Capture One profile design process they indeed do the adjustments manually, and they work a long time with it -- for their medium format cameras. Spending many hours or even weeks on a single set of profiles for each camera is of course not reasonable so I assume they use more automatic methods for the consumer cameras, and do their costly manual work only with their medium format cameras.
What I don't know yet is once you've designed a successful look how easily that can be re-used on a new camera. Hopefully much can indeed be re-used and then it's not so much work supporting a new camera. If you have to go through the whole process again though, then you could only focus that effort on the cameras you really care about.
If you use Canon or Nikon's own software for their own cameras you will most likely get access to more worked-through profiles than you get in Adobe Lightroom or Capture One. However there's a lot of taste involved too of course so the profile that has got the most attention may still not match your taste best.
There's a lot of myths and bias in there too. I've seen people praise the "color accuracy" of the medium format cameras, out of the box. But you certainly don't need to have "golden eyes" to see that the profiles have no intention to produce "accurate color". Simple A/B comparison between real objects and captured image in a calibrated system and you see hue shifts, lightness shifts, saturation shifts. Intentionally designed of course, they provide a canned look. The simplest demonstration of this is comparing Leaf and Phase One cameras that share the same sensors, yet with the default profiles produce quite different looks.
I haven't studied Phase One cameras that much, except for the P45+, but I did own a Leaf back. While providing a pleasing look I think they added too much of a look, but they had their oddly name "ProPhoto" mode that was quite neutral. Now I have a Hasselblad and I think it's about the best I've seen so far in the balance between realism and a look, I'd say it's about "95%" realistic, and very subtle looks added, except for high saturation colors which are strongly gamut mapped (desaturated to fit in a smaller gamut, which was how you did things before computers where able to do realtime gamut mapping).
Leaf's strong looks seems to work very effectively as a lock-in though. I've recently been in contact with a user that for external business reasons changed from Leaf Credo to Phase One IQ, and now they have very real issues to achieve the color they want as they've been used to start off with the Leaf canned looks. I think the strongest reasons to develop a workflow with your own custom profiles is simply to avoid this type of lock-in.
I haven't really figured out Adobe. They allow embedding a curve in their profiles, but if you do the colors will be garish, unless you pre-compensate with a LUT. In my own profiles this pre-compensation becomes pretty strong and requires a large LUT, but Adobe's own profiles while pre-compensating some it does it to much lesser extent. That is quite much of this curve look is left in the profile, and I think that is one of the more important reasons that Adobe's color is considered not that good by many. Capture One has a split approach, some of the curve is implemented in the LUT, and some is added on the side. The reason is to counter-act the negative color appearance side effects of the curve. I think it's quite obvious that Capture One have had a better understanding of the color appearance side effects of a curve than Adobe has, and has implemented a better solution in their profiles, at least in the profiles I've studied. There's no problem to fix in Adobe's DNG profiles if you want to, but Adobe doesn't really do it.