I believe the reviews you have been reading refers to all the extra software that PC manufactures add to the standard Windows installation. It usually consist of things like face recognition software, back up and restore software, anti-virus and firewall software, manufacturer branded games, and anything else that the manufacturer has decided the end user will need for a "good computing experience'.
The amount of the additional software can be mind boggling at times, and it goes way beyond a simple welcome screen.
I feel the Welcome app should stay, as it is a great way to introduce some basic features of Ubuntu Mate to new users, especially to those who are new to Linux. Plus if it's not desired to run at login, one only needs to uncheck the check box.