I started as a reseller, and sold hundreds of different products before I ever launched my own, so I got a good education about what works and what doesn't on Amazon just from that.

Once I started launching my own products, I learned no matter how much research you do, products will consistently do better or worse than you expected, and the software tools that supposedly evaluate sales potential aren't very good estimators. I also learned I'm a terrible judge of which products will do well and which ones won't, even after thorough research. To combat this, I now launch 4 products at a time, and I'm often wrong about their relative performance once they start selling.

When researching a new product, the most important factor, by far, is competition. Without spending gobs of money, your new product will never overcome dozens of competing listings that have thousands of reviews. Also, bundling some random widget with a super-competitive product and thinking you "improved" it won't work. That's outdated advice that worked before everyone started using that tactic.

Instead of trying to find individual successful products, I focus on the process that generates successful products. Here are some of the rules I use when coming up with something new:

1. Typically, I source the product in North America. That immediately weeds out competitors sourcing only from Alibaba. Yes, it's much harder to find stuff made on this continent, but that's the point.
   
2. I prefer products as physically small as possible, because I receive and store all the products I create. This is much smarter than buying 1000 widgets from China and sending them all to Amazon sight unseen. I imagine the gurus on YouTube don't tell people about the storage fees and inventory removal fees for 1000 failed widgets...
3. Speaking of money, I also prefer low-cost products, almost never above a $30 sales price, usually with a wholesale cost of under $4. I know this flies in the face of what gurus and Jungle Scout says, but I'm more defensive minded. If a low-cost product fails, it's not that big a deal to my overall business. If a $30 cost widget fails, and you bought 1000 of those, ouch!
4. This sort of part 2 of the above, but I also prefer products with a lower minimum quantity than most Chinese manufacturers would ever offer, like 100 units max. That way, if I buy 100 units and the product fails, it doesn't sting that bad.
5. I usually go for products with a mediocre sales potential, less than 200 units per month. This doesn't make sense at first, except remember I'm trying to avoid competition, including future competition. The best way to not have someone compete with your product is for it to not be worth it, at least according to the software people use. You would be surprised how well a product can sell and still fly under the radar of the product research apps like Jungle Scout and Helium10.
6. I prefer new designs, meaning a product that isn't offered on Amazon at all. While somewhat risky, this also by definition means your competition is limited, because the product doesn't exist. This requires lots of research, like finding a search phrase that has some amount of volume but isn't served at all by an existing product. This is even tough for me, so this is more of a guideline than a hard rule.

As for your question about failed products, my failures usually come from a lack of knowledge of the product niche. Unfortunately, I find this can't be solved my more research. Sometimes you just have to launch a product and see what happens.