The vast majority of software will run under Linux in one way or another.
Linux did not gain more ground precisely because of this. I believe the devil is in the one way or another part. You probably can get almost any software to work, but it is not a "double-click this" level of effort.
I usually forget what software I started to install (on CentOS, typically) by the time the 5th library had to be added.
The only major exceptions are games, and even many of those will work.
Do you mean natively or through Wine (or such)? With all of the fun of determining the required configuration settings online?
The user interface in Ubuntu is "good enough for grandma".
It is really time for someone to do a proper study because I have difficulties believing that statement -- but that's all too anecdotal.