I avoid the use of all general-purpose computer systems for live performance having seen far too many of all brands (ranging from vintage Compaqs to recent Macs) work flawlessly up until showtime and then randomly conk out with some bizarre failure moments before or even during the actual performance, despite doing many run-throughs and resets flawlessly even the same day.
To say that I’m cynical about the state of GP computing is a massive understatement, and I’m with you - I wouldn’t trust any recently manufactured system for live duties until it had performed flawlessly for a while in a practice/studio setting and gotten into the reliable phase of the famous ‘bathtub’ curve as we engineers call it. But that goes for any definition of ‘recent’ across any year, I’m not singling out the 2018s or the upcoming 2019s or any brand… I just wouldn’t use a recently purchased system of any kind for live shows until I had gained confidence in it with my setup.
The proliferation of A/B instant-failover rigs (like the iConnectivity stuff and Radial’s A/B switcher) in today’s touring rigs should speak to this too… even proven touring workhorse systems fail spontaneously, almost always mid-show (and never in rehearsal, of course!). So if you’re looking for reliability in performance never look to any specific brand, but look to a redundant and time-proven rig, regardless of your system preferences and hardware choices.
Edit: and to stay on topic, I do say that if you’re going to get a Mac now, get a new 2018 system and don’t get a 2016/2017, because of the improved keyboards and there will be support and security updates for that much longer, and security updates are going to be an increasingly important concern in the upcoming years. Plus the new chips have some very nice improvements in audio/media processing that are nice to lock in versus the previous generation - there was a solid bump in capability the past year or so and I don’t expect 2019/2020 to offer significant performance increases, we’ve locked in the current round of ‘next gen’ for a little while, I think. But if your 2013-2015 system is working fine, I would also advise you not to feel a need to upgrade unless you have a very specific reason to do so and not simply “it would be nice”.