The largest issue with individualism is that it treats individual behavior as something which can be disconnected from a collective whole, and this is simply false. Not only false because we're always and have always, as a species, been part of a collective whole, but false insofar as it denies what a collective even is - a collection of individuals. When somebody says "We need to get these hundred million children some healthcare or they will die of Polio," it's probably easier to be calm in that situation than in the situation of watching one man die of polio because one man is easier to humanize than a hundred million - we simply can't imagine that many people suffering that badly at once because it would destroy our sanity to even try to do so. The market anarchist crowd especially loves preaching about a specific type of heightened individualism, annoying to a detriment and intentionally obtuse. What's ignored here is that markets are, in the end, nothing more than a method for enforcing collective wills. Individualism as a whole, however, goes even deeper than this. Insofar as it is methodology, it's useless for understanding the world because it's simply incapable of reaching levels of abstraction required for analyzing human societies, history, etc. We can't view history as simply "the adventures of the abstract individual through time" because said individual doesn't exist - individuals are shaped by collective forces, by powers above them (for good or for ill), and there's simply not much wiggle-room out of this. Insofar as it is a revolutionary method, it doesn't work. There's a reason even Anarchists (a self-proclaimed 'individualist' ideology) opt for collective action nowadays rather than the boring and banal heroisms of totally randomized, personal acts of violence - it works better for popularizing their movement and ideals. In terms of social analysis and mobilizing forces of revolt in society - collectivism, collective action, and not conceiving of things in terms of the individual, not individualism, have been shown to work, and that is what actually matters in those contexts. But what of liberty? Does not the liberty of the individual precede and even surpass that of the collective, necessarily? Is it true that if one is not free then none are free? In what sense, precisely? We remove the capital from the capitalist and this does take away freedoms, his liberty is gone, whether we acknowledge liberty brought from privilege as "genuine liberty" or not is irrelevant - because he does. The collective comes first in politics because, at the end of the day, the purpose of politics, especially in the context of radical liberation struggles, prerequisites the freeing of society as a whole - only through collective liberation can the individual even conceive of being free, and for both people this is as true materially as it is in terms of their conscious thoughts.

The shorter version here is a largely utilitarian argument: I am against individualism because it is a failure of methodology, and even the far-right recognizes this fact when shit hits the fan, whether in the form of Reaganism or Nazism, both involved mobilization and collective action and support by a majority of the populace against those whom the rightist ideologies opposed. Likewise, however, in terms of freedom - whereas the individual says "If one of us is not free, then all of us aren't free," I say, "No, you may be free at the cost of another, you just don't want to think about it in those terms." Other arguments, of course, could go a bit further - the fact that individualism in its vulgar forms rejects the existence of society as a whole (see: Thatcher), the fact that individualists have a long history of, when push comes to shove, being more adjacent to Fascism than any form of Leftism (with the exception of the Anarchists, of course), the fact that the distinction between individual and collective requires an ignorance of how an individual is constructed and constituted, etc., etc., I think it's all out there. Individualists have had good things to say, but their individualism wasn't among those 'good things'.