I have a mid-2012 MacBook Pro - aka the last "Unibody" model that they manufacturered until 2016 and the last where they don't consider DIY memory and drive upgrades to be unauthorized modifications.
For various reasons I was lazy about upgrading anything on it, including 500 GB 5400 RPM drive and the 4GB standard memory. I was afraid of touching anything inside because the cover had screws. Then I suffered a lock up condition (probably from a bad USB cable insertion) where it was clearly not working but also clearly not turned off, and I couldn't turn it off with any buttons. So I took my chances by opening it up and then pulling the battery connector and putting it back in. Did the trick. It was in some sort of zombie state where it wasn't really on or off. But then I saw what was inside and figured it wouldn't be too hard to replace the memory or drive.
I did replace the drive once with a 750 GB 7200 RPM hard drive after the factory drive failed, but soon after I got a 512 GB SanDisk Ultra 3D (that capacity was a Best Buy exclusive) and everything got so much better even with only 4 GB of memory. I've talked to several people who indicated that there's no more effective performance upgrade. Even if it was strapped for memory, the SSD could actually partially replace the memory - I think it's called virtual memory or something like that. I did eventually max out the memory but it didn't come close to improving the performance compared to the SSD.