There's no problem with worshiping Sri Lakshmi. Srila Prabhupada explained that when we worship Radha and Krishna with awe and reverence—according to strict standards—we actually worship Lakshmi-Narayana. Lakshmi is an expansion of Sri Radha. The problem—and the condemnation of the acharyas—comes when we worship Sri Lakshmiji in order to achieve some material prosperity.
Generally pictures of Sri Lakshmi depict her with coins coming from Her hands. She is considered the "goddess of wealth." Seeing Her this way, worshiping Her as a demigoddess, and asking Her for material wealth are all "condemned by the previous acharyas."
Sri Lakshmiji is the consort of Lord Narayana—She will bring us into His favor if we serve Her nicely—but if we ask Her for service, especially materialistic service, we're missing the point. The point of life is to worship Lord Krishna and go back to the spiritual world. The purpose of worship is to please the Person being worshiped—not to increase the material prosperity of the worshiper.