Trickle down spending never seems to trickle any further than the wealthy class. As someone else has mentioned the increase in deficit spending ans national debt was another concern.
"Reaganomics" was also called "Trickle-down spending", meaning that money spent on government programs at the top of the economy would theoretically trickle down to the workers at the bottom, and that tax cuts to wealthy people, businesses and investors would show up in increased wages and hiring. This idea is based more or less on the theories of John Maynard Keynes, a prominent British economist in the 1930s.
Some people argued that it didn't trickle down far enough, that the gap on Reagan's watch between the rich and the poor got worse, and poverty increased at the same time as wealth did. It was also criticized because in order to finance a trillion dollar military buildup and other government programs that stimulated the economy, we fell into record deficit spending, and the national debt tripled during Reagan's two terms.
I would say that the major criticism of Reaganomics from liberals is that it cut spending on domestic programs. They would argue that it ended up hurting the people who were most vulnerable and in need. There came to be more people living in poverty even as the country as a whole became wealthier. Liberals might also criticize Reagan's policies for the fact that the US trade deficit went up during his time in office.
To the extent that conservatives criticize Reagan, they say that his economic policies did not cut government spending enough. They point out that the national debt increased during his time in office.