The fact of the matter there are NO PROBLEMS with YOSEMITE at all. When users do experience difficulties it is from trying to upgrade from older Operating Systems.
1st users fail to do there homework and there older machines simply don't have enough RAM.
2nd they get mad because there software will not work on the newer Operating Systems.
When upgrading to a newer software be sure you find out what the bare minimum System Requirements are for the upgrade.
After doing this and always make sure that you make sure you keep all of your SOFTWARE UP TO DATE. This is also one of the culprits when people have issues and are not happy about upgrading older machines to the newer Operating Systems.
Hopefully this has helped, Cheers and Good Luck
Don Morgan