I have a Mac Pro tower mid 2011, Xeon two quad cores. After installing Mac Mojave my computer kept restarting then not starting. I tried everything from replacing CR 2032 battery, to changing my graphics card, to pushing all sorts of button combinations, testing my Power supply - it was working fine, to taking out the CR2032 and letting the computer sit without power for 48 hours( which has worked for me before on earlier towers and MacBookPro, lately.
As a last resort I put in a Windows 10 hard drive( having seen a couple of Mac Book Pros using Windows 10) and I am happy to report my Mac is working perfectly.
I had some problems after installing High Sierra a few months ago and I believe that Apple is inadvertently damaging computers which it says are compatible to it’s new systems but is not. I doubt very much that Apple is going through the process of dragging out older Macs and testing their newer systems on them one at a time. I suspect that that they merely applying a formula with regard to basic common elements( like does the computer use DDR3, is it an Intel chip based machine, can it support adequate graphics and so on.) I also think that they have upgraded my firmware incorrectly.
I dislike Windows generally but I spent 800$ for this machine - Windows isn’t that hard to use!