Since out of the three Injectors give me the worst shave I'd have to say they have the highest learning curve...Although I couldn't vote this way, the learning curve for DE and SE were both very intuitive and similar for me...
...and compared to learning to competently wield a straight razor, all 3 are considerably lower curved. Single-edged razors are probably to me the easiest to figure out, so I chose SEs. I've very little experience with injectors.
This! Even the amount of years is about right lol. My unsolvable issue with cartridges is that my beard is too thick for them. Two inches into a pull and they start to clog. This left me scratching over the same area multiple times and spending more time unclogging than shaving. I always kept a dedicated toothbrush on hand to dig my beard out from between the blades. Having said that, never really had any technical issues with cartridges. DE shaving was a relief from the first try, and I had the hang of the technique enough to enjoy it in about two shaves. Relax, inhale the nice scents, go easy on the pressure, keep a consistent angle and no worries....
Straights are my next project. Expecting those to take a bit longer to get the hang of. I have already been perusing the Whipped Dog website and spending entirely too much time lurking in the TSD Straight Razor forum...
Injectors.....But Type I and below. Above the Type I (Type G & E) can take a little more care. The Type A & B have a very strange and at times hard to find angle. If you don't hit the sweet spot they will either burn you or do absolutely nothing. In my experiences with them.
Super Speeds & Techs are what I learned on as a youngster and I don't recall any particularly difficult learning curve, but the mild nature of those razors was definitely a good thing for me. One of these days, I swear, I'll get around to trying some of those beautiful butterscotch handled Schicks as well.
I don't think any are inherently more difficult. Whichever one you start with will have the greatest learning curve, using the others will build on whatever you've already learned with the first.
Injectors - IF any of the currently available blades work for you. Nothing I have found is sharp enough, so I can never quite get a satisfying shave. But they are definitely a good design!
I gave up cartridges and went with a Star SE that was my grandfathers. The biggest learning curve with it was learning where to find decent blades. As for my r89 DE, I seem to have had more trouble with the angles and blades than the SE. As for pressure, I can put more with my R89 than my Star.
I don't think their is much of any difference between any of them. No pressure is no pressure. Straights are a different story.