At the risk of sounding clueless, I think a good workout app can never be comprehensive and one-size-fits-all. I think they should be based around a clear and well-articulated philosophy and specific goals which lean towards highly opinionated.
Maybe you want to build upon dat ass or strengthen your hips to reduce injury. These are wildly different goals. One app should not aspire to do both meaningfully. However, an app focused on dat ass or injury prevention programs? Stellar ideas I think, and they can be executed on effectively.
Targeting a body part is also a weird concept. Target it to do what, and how? Why?
My goto after 15 years is lifting at a relatively low weight (kettlebells and barbell) and bodyweight progressions using tempo patterns. I used to go heavy and do fewer reps and sets because I wanted to maximize strength and minimize time working out, but learned over time that you really do max out strength and neglect a lot of other things. I was getting really strong for some random dude sitting at a desk all day, but try as I might, I was losing mobility and hurting myself despite an intense focus on form and checking in on how I was feeling and getting experienced people to review my program, lifts, and progression. To get to that point I had to learn a lot and make a ton of mistakes already.
Tempo lets me focus on form better, find compound movements I can move through safely in broader ranges of motion, and get my heart rate up higher for more of my session rather than in exhausting bursts. I injure myself less, my mobility and strength are better rounded, and I don't need to eat like a bear to prevent wasted effort. I love it. I've turned into a fat ass recently but that's unrelated; I'd probably be more of a fat ass had I kept lifting hard and eating harder. The point is, what works well for me (and others without a doubt) is certainly not something that I'd get from most generic workout apps I've encountered. And if I did, it would have been accidental. There is usually no clear reason or philosophy behind why you're doing what you're doing; it's just another workout in the database piped into your program. Just do it, because do it.
I think the programming I do today a relatively nuanced fitness foundation, and apps rarely ever touch on these matters or how to make decisions about them. They treat exercise like a very static, linear thing. Pick the movement, pick the gear, do the thing, you exercised. But bodies are so dynamic, movements yield different results under different loads, and people have very broad ranges of goals. It isn't good enough.
Want to get huge? There are lots of ways to do that. Want to do it based on the equipment you've got? Sure, we can narrow it down a bit now. Do you have any physical limitations? We can narrow it down even more. I think you can make a useful app out of this singular goal (and some people have, I think). But a random "what gear do you have and which muscles do you want to work" app is kind of like... I mean, what does working the muscle mean? What are you actually going to end up doing, and why?
I also agree completely on the point about potential harm. I think this is another reason to build an app around very specific methodologies and goals. There's more opportunity to hone in on great explanations of safe form and equipment usage, explain methods, and generally get the beginner acquainted with effective implementation of the movements and overall strategies.
But again, I'm quite literally a fat ass and I don't really know much about this stuff. I have lifted some weights, done a bad job at it a lot for a long time, and found ways in which it worked here and there. My sense is that we need to be specific and targeted, not all-encompassing, if we want to create great programs.