Comprehension

Hello

It's a passage from a book in which an author writes about a power of suggestion. He says:

There is not space for me to do more than mention other ways by which we are unconsciously affected and influenced by suggestion. Newspaper and magazine advertising of patent medicines are most potent and powerful mediums of suggestion. We have already mentioned the effect of being told day by day, in our paper and by means of hoardings, to take a certain make of pills. But modern ideas of advertising worthless nostrums and harmful drugs leave such methods of advertising far behind as regards suggestive force and value. Pictures of people sneezing, and of others doubled up with painful backs, can have only one effect, and that is to make people imagine that they possess the ailment described.

There is something not clear to me, but maybe it's just to me.

First he says that newspapers and magazines are the most potent and powerful mediums of suggestion and that they're very successful in advertising different patent medicines. Then he adds that earlier in the book he had already mentioned how the advertisements in newspapers and on hoardings (billboards) make us buy certain products. AND THEN he says that "modern ideas of advertising worthless nostrums and harmful drugs leave such methods (which methods? newspapers, magazines and billboards?) of advertising far behind as regards their suggestive force and value".

If in this sentence he says that there are far better methods of advertising than just by newspapers and magazines and hoardings, why did he say first "they are most potent and powerful methods of suggestion"?

It's not clear to me.