As far as their search business, the "harm" is that Google has too much power, influence, control, etc. over the delivery and manipulation of content/information on the internet. They also have too much power, influence, control, etc. over search ad marketing including pricing/costs. There are also the allegedly illegal search default agreements with companies like Apple which are used to help Google maintain its "monopoly" in search.
Google as a company is also dominant in other areas like browsers (witch Chrome) and mobile OS (with Android) and, one way or another, uses all to help them maintain dominance in each through default settings, pre-installation, bundling, etc.