Full Idea
Leibniz's three objections to occasionalism are: it disturbs the concept of laws of nature used in physics; it introduces perpetual miracles; and it doesn't recognise activity of substances (leading to the Spinozan heresy that God is the only substance).
Gist of Idea
Occasionalism give a false view of natural laws, miracles, and substances
Source
report of Gottfried Leibniz (works [1690]) by Nicholas Jolley - Leibniz Ch.2
Book Reference
Jolley,Nicholas: 'Leibniz' [Routledge 2005], p.64
A Reaction
I wonder what would happen if, within the viewpoint of occasionalism, God suddenly packed up and abandoned his job? Presumably the world wouldn't disappear, so there would still be substances, but passive ones, in chaos.