1st reason would be the massive amounts of misinformation that internet forums are responsible for. Ableton Live is capable of doing whatever you need it to do seeing it's more powerful than what people 15 years ago were using to make music. One problem some people come across is the PDC issue when running 3rd party plugins so that might be an issue if you plan on using tons of Fabfliter or waves plugins when mastering. I've never encountered PDC problems because I stick with stock plugins. Limit yourself and amazing things will happen, I believe.
2... Live was not meant for mastering albums. That's not it's purpose and it's been repeated by the big dogs at Ableton. People who produce shouldn't be mastering their own music anyway.