If anyone asks me should he/she get a new MBP, my advice is: no, no and NO.
Out of first 16 MBPs in my company, 9 had keyboard failures in the first 5 months. 2017 version wasn't any better, so MBP got banned from being purchased at all.
2009-2015, not even one keyboard failure. There is some context to you
Appreciated. You should be aware however that the 2018 models seem to have for the most part fixed the keyboard issues in the v3 iteration of the butterfly keyboard, thanks to the new membrane below the keys, so if you're stacking the 2015 models vs the 2018 ones, I don't think it's very relevant anymore.
Aside from that, the 2015 model is inferior in almost every way compared to the 2018 one, and far inferior in a number of ways. The 2018 models have a significantly better display (larger color space, higher brightness, TrueTone, ...), their speakers are leagues ahead the 2015 models, performance saw a huge spike especially with the 2018 models, much faster SSD and RAM speeds, a huge increase in portability and decrease in size/weight, Touch ID, trackpad is far larger, a port selection that can objectively do more (even though it's slightly less convenient at times), can be charged from both sides with a universal charging standard, many more smaller but still handy advantages like "Hey Siri", Bluetooth 5.0, the T2 chip for a variety of minor but meaningful improvements like more security, faster SSD access and much faster HEVC encoding... and so on and on.
The only wild cards really are the keyboard (some love it, some hate it, for many people it doesn't matter, but the reliability issues are mostly a thing of the past) and the Touch Bar (same thing: some love it, some hate it and many people don't mind either way, but it's way more versatile in the long run); everything else is almost universally considered an improvement (give or take the port selection). If we're talking about a, say, 400€ price difference between the 2015 models or the equivalent 2018 ones then it's absolutely a no-brainer for me to advise people to buy the new models that will be a better experience for them in almost every way and very likely last them far longer down the road.
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As far as bulky goes, new MBP throttle a lot. So if someone needs performance, avoid new MBP completely, even though they are slim and sexy, no denying that.
The 2018 models still show huge performance gains over the 2017 (and even more so 2015) models, so advising people to stay away from the newer models for this very reason but not from the old ones seems a bit like cherry-picking when they are still a far better choice from a performance point-of-view than the 2015 MBPs.