Angels don’t have any power on their own. It doesn’t matter if God unlocks the door and the angel just tells about it, or if the angel unlocked the door because God working through the angel gave angel the power to do so.
Angels also do things other than deliver messages. For example, St. Michael the Archangel fights Satan. When St. Michael the Archangel fights Satan, he’s not doing that under his own power. He’s doing it “by the power of God”, as the Prayer to St. Michael says.
Whenever God sends anyone as a messenger, or to actually do some task, whether it’s an angel, or an apparition of the Virgin Mary, or an apparition of a saint, God is the one with the ultimate power to do whatever task needs to be done, and anything the angel, Virgin Mary, or saint does is “by the power of God”.
I’m not sure why people always seem to have concerns with an angel/ Mary/ saint doing something, as God is ultimately in charge every time.