Scientific knowledge is based on observation, measurement and experiment, thus on experience, not on authority. Much spiritual teaching is based on the authority of scriptures or gurus. If it can be established that a guru knows whereof he speaks, then it makes sense to accept his authority at least provisionally although many gurus claim more than merely provisional authority. Buddhist spiritual teaching is a guide to practice and is to be confirmed or disconfirmed by individual experience. On this basis so far, I accept the teaching of karma (action and consequences) though not of individual rebirth. Zen monks authoritatively tell us what the teaching is but we are not obliged to accept the teaching on their authority.
Utopian socialism is an idea based not on authority but on abstract reasoning. It would be good if the economy were co-operative, not competitive etc. Utopian socialists can only argue that their idea is better than the current reality whereas Marxist theory guides the kind of political practice that potentially builds socialism. Effective arguments for socialism occur in the context of struggle against aspects of the capitalist system and of its state apparatus. All such struggles potentially challenge the system and demonstrate how to overthrow it. Thus, Marxism is "scientific socialism", to be confirmed or disconfirmed by collective experience. Marxism would be disproved either by a crisis-free capitalism or by the continued oppression of social minorities in a genuinely socialist, as opposed to state capitalist, economy. Also, Marxist theory has developed in the light of subsequent experience. Such development continues to explain capitalism, not accommodate to it.
Buddhism contrasts with authoritarian and theistic spiritual traditions. Marxism contrasts with utopian and reformist socialist movements. Although neither Buddhist teaching nor Marxist theory achieves scientific precision, they are our closest approaches to scientific spirituality and socialism.