Dorman idler pulleys for domestic applications are of good quality, in my experience. You have to have the ability to look at a part and evaluate it or have someone you can ask who knows, that's pretty much the bottom line.
The place where I buy most of my auto and truck parts will tell me honestly whether this brand or that is junk, even if they offer it. They will tell me honestly how many they've sold in the last 5 years and how many have come back, (it's right there on their computer screen in front of them).
I buy a LOT of auto parts. I used to buy a lot of Porsche parts from Pelican when I had an air-cooled 911 many years ago, they did not sell cheap aftermarket anything in those days. It was all Porsche dealer parts or actual original parts, (as opposed to OEM, which means something entirely different). I could type a whole page on auto parts; the industry and it's relationship to globalism and the decline of manufacturing in the 1st world, etc., etc...
It's a subject near and dear to me. To make a long story short, most replacement parts are not what they used to be and are not made where they used to be. Most Bosch parts are made in the 3rd world or Turkey. 20 years ago, they were almost exclusively made in Germany. You need to access the best parts you can for the best price. The good news is that the internet has made the auto parts business an absolute cut-throat, price competitive industry where I buy the best parts available today for pennies over cost in most instances.
I generally do not buy parts from Autozone, (NAPA is a lot better), but AZ's business model is appealing because of the warranty. What they do is sell absolute crap parts that were made in someone's bathtub in Tijuana but they warranty them for life. There are AZ stores everywhere in the U.S., so when that crap alternator or PS pump takes a dump in the middle of Nebraska, you simply turn it in for another one at the nearest AZ.
They know that their parts are garbage and will fail ~20% of the time, they don't care. It's built into the business model. If I'm rebuilding alternators for $8.00 a piece including shipping and can sell 2 million of them @ $79 each, wtf do I care if I have to give you another one @ $8.00 once in a while? It's not like I'm giving the $79 back.
So to summarize, lifetime warrantees are great but they do not correlate to quality, sometimes just the opposite. Things that are attached to the front of an engine like idler pulleys and alternators are easy to replace and therefore commonly sold to DIY car owners, so if you have to replace them every year or two, big deal.