The nature of modern electronics products is that they'll either fail almost immediately, or long after they were put into service. Essentially, if it doesn't blow up when you first plug it in, you'll be well past the first 7 days before something goes wrong (retailers wouldn't make that promise if there was a high probability of having to make good).
For example, it can be months before a sub-standard bearing in a fan or hard drive is going to create enough noise to be noticeable/measurable outside of a laboratory environment (and mechanical systems have the highest probability of failure). So even if a particular model of cooling fan turned out to be more prone to premature failure than another, it could be quite a while before you have a hint that your fan, in your computer, was going to be one of the 0.001% that fails way too soon.
A single defect does not a lemon make. The common definition of lemon is a product with multiple, serious defects. It's rare for anyone to know they have a true lemon within the first seven days. Cars can be identified as lemons because even major defects are likely to be repaired at the dealership - the owner has a chance to see other defects emerge over time. A high percentage of consumer electronics products are replaced, rather than repaired, so even if the item did have several defects waiting to be discovered, the original owner isn't likely to encounter more than one of them.
Then, of course, there's the echo chamber that is the Web. A handful of first-hand experiences are seen and repeated second-hand, third-hand...each echo larger than the last. "I've heard that...." (Why do you think they call it Mac Rumors?) There's simply no statistical validity.
crows list of possible problems has a common thread - they're all pretty obvious. Inspect the case, scrutinize the display, listen carefully. If you haven't noticed any of them on your own, they're easy enough to find. (And note that "normal" fan noise for one model can be abnormal in another - even the location of fan vents will affect your perception of noise.) Problem with RAM modules not connecting? Apple Menu > About This Mac. Either installed RAM matches what you bought, or not. Troubleshoot if necessary. Considering that memory modules and their mating connectors are stock items that are found in dozens of models... your odds of a problem are likely to be no better or worse than for any other model you buy.
So, you've already had enough time to go through the checklist. How'd yours do?