If you have an old Macbook or iMac that will not upgrade to the latest OS. There is a good chance that we can “Hack it” to upgrade beyond its recommended specs. With some clear expectations of how the Mac may function
May be slower
Anytime you upgrade the software it does use up more resources. This does mean that some computers can become very slow. So depending on how you use your iMac or Macbook you may consider not upgrading to a newer OS beyond it specs.
That said I am writing this on a 2015 Macbook Pro with 16 gigs of RAM running Ventura and it is running fine. Other computers in my shop however are not so lucky. Some have slowed done quite a bit. But those computers are only being used for training so performance is not as important.
Old software may not work
If you are using a very specific software that was once used on MacOS Leopard or other old OS versions. There is a good chance that that software will not run on the new OS at all. While some people may think this is planned obsolesces it’s honestly not the case. T
So if you decide that you need to upgrade your software you should take a look at all the software you use and decide if there is any software that would be devastating if it failed to run after an upgrade. While I try to keep a list of software that will not upgrade it is impossible to have a definitive list.
A good example of this is MS Entourage. This was software that was either hated or loved by its users and was eventually abandoned by Microsoft a few years back. There are other bits of software that will do the same job but entourage has been known to cause problems moving data to Outlook etc.