“ And it shall come about that as all the good things have come on you which YHWH has promised, so shall YHWH bring on you all evil things until he has destroyed you from off this good land which the YHWH your God has given you.”
This was the other side of His promise. Just as He had fulfilled His promise in bringing good things on them, so He would fulfil His promise to bring evil things on them if they failed to obey Him and learned the ways of the Canaanites. They had received the land because they were His people and the Canaanites were evil, but if they turned to the ways of the Canaanites they too would perish from the land. The ‘evil things' are described in Deuteronomy 28:15-68, and they make dreadful reading. To be favoured is to render oneself liable to the greater punishment.