I don't have a 2018, I'm not ok with the risk
In terms of probability, I'm not an expert, but let's say that the failure rate is 10%. I don't know what the industry standard is and I'd guess 10% is high for the industry, but let's use that. That means that having a single failure is a 1/10 shot. Having two consecutive failures is then 1/100 (1%). Having three consecutive failures if 1/1000. Or if it's a 5% failure rate, then that's 1/20, 1/200, 1/2000 respectively. Whereas the odds of having one failure and then having the second one be excluded from the "failure pool" (ie, a good unit) would be 90/100 or 90%. Does it seem like 90% of the folks with the 2018 Pro problems have had a replacement unit fix the issue? I'm not sure, but my gut feel is that it's not close to that.
Neither the 1/100 shot of two consecutive failures, nor the 90/100 shot of having the replacement machine resolve the issue - neither of those pass the sniff test to me, for what that's worth (admittedly very little).
If the failure rate was 30% as opposed to 10%, then the odds would be 3/10 of having a failure out of the store, and the the odds of the second unit having a failure would be 9% (vs 1%). 50% failure rate would be 25/100 (25%) of having a faulty machine replaced with another faulty machine.
So again, totally possible my math is wrong (I'm not a probability nerd and I'm wading in to a bottle of wine), but the repeat failures to me is a red flag that the rate is higher. Those instances where people had a problem and then the replacement also had a problem.
I'm sure there is someone who knows far more about probability than me who can address this better.
I haven't kept up very closely and I could be totally wrong. Maybe I should pick one up and try it for myself!
Totally agree with your AppleCare comments. I didn't opt for it on my 2015 but on a 2018 I would. I'm actually a little surprised that it hasn't been hiked, considering how picky some folks are.
Your probability is a little bit off, and the results depend on how you want to extrapolate the data.
You were correct for the 10% failure rate. There is a 1/10th chance of something happening once, a 1/100th chance of it happening twice, 1/1000th for a third time in a row; purely by chance.
If it is a 5% failure rate, it goes from 1/20th the first time, to 1/400th the second time, and finally 1/8000th the third time.
That is a large reason behind why companies want their failure rates to be as low as possible. A 5% decrease in failures provides an exponential decrease in the probability of multiple failures. For example, that 5% decrease in failures led to a four time lower chance of a single replacement having problems, and an eight time lower probability of a customer suffering a failure on a second replacement or exchanged product. This is assuming that you are running on controlled variables and everything is based purely on chance.
As you can imagine, there are limitations to this. The largest comes from the fact that the variables are uncontrolled. For example, let's say that Apple produces a run of 10,000 MacBook Pros with faulty speakers in a row. Then those 10,000 MacBook Pros all get sent to one store. Joe Blow comes in and buys a MacBook Pro and finds the speaker issue. He takes it in and exchanges it for another laptop. Well, come to find out, that one sucks too. Joe Blow tries it a third time with the same result. Regardless of probability, if the entire batch is contaminated, until Joe Blow finds a different set of stock to pull from with no faults, or the store runs out of faulty stock and is replaced with new stock, he's going to continue to get the **** end of the stick. Theoretically speaking, Joe Blow could go through 9,999 replacement laptops before getting one that worked correctly. This is often why you will see repair programs only covering a specific set of serial numbers, as Apple is able to backtrack those repairs and complaints to narrow them down to a specific set of production criteria. This is also why I think that a lot of people swapping computers wind up with the same issue that they were trying to escape to begin with. In the big scheme of things, while the probability of failures happening on multiple devices is small, the reality of production and distribution channels skews that probability drastically if there is a batch of faulty equipment sent out.
On a much larger scale, what happens is we wind up with Chicken Little level cries of "The Sky is Falling" as users flood the forum to find solutions or complain about their issues whenever a big batch of faulty equipment goes out; exceedingly rare if I were to guess. 95%+ get a laptop that works well and only a small percentage of them wind up on the forums.
You also have to account for the end-user as well. Some end-users find fault in everything and will obsess over every possible imperfection and keep swapping computers until they find one that has no imperfections in any possible way. I have heard stories of people swapping laptops because they had a key that they thought was "clicker" than the one next to it. Just asinine stupidity and something that probably deserves medical intervention to treat. This type of behavior comprises a majority of the replacements, swaps, and moaning that I see on the forums between the MacBook Pro and the iPhone. Some issues, like the screen flickering that others have reported, are caused by a very specific set of circumstances that are a result of a larger hardware failure or (much more likely) a software bug / failure that swapping units won't do anything to solve. That is largely dependent upon how individuals use their computers. For example, the screen flickering is largely associated with external monitor use. Mine might exhibit that same behavior that everyone else is complaining about, but I don't use my computer in that way (nor would I likely ever). Since I never experience the issue or have to deal with it, my experience is different than someone else, even though my computer could very well cause the exact same issue.
Anyway, the best advice I can give you is to do what serves you the best. Apple takes care of the customers in a much better way than many other retailers do. If you buy one and need to return it, it's not too big of a deal. I would highly recommend trying one out if you have the desire, need, or money burning a hole in your pocket. I have no regrets and mine seems to be working absolutely perfectly in every regard. As you mentioned, I would highly recommend purchasing AppleCare+ for the computer if you plan on keeping it or using for more than one year. Once that time is up, pray that the reliability holds up. The repair options are so God damn expensive these days that it is truly almost better to just buy a new computer with a warranty than it is to repair the old one and hope for the best.
I would also recommend purchasing one that requires a custom configuration and build. That ensures that your laptop is amongst the latest build and, in my opinion, has a lower chance of having early run issues that others might have while the stores are clearing out their stock of first run examples.