Experimental evidence suggests that Marx was right.
In Capital, Marx describes the selling of labour power thus:
He, who before was the money-owner, now strides in front as capitalist; the possessor of labour-power follows as his labourer. The one with an air of importance, smirking, intent on business; the other, timid and holding back, like one who is bringing his own hide to market and has nothing to expect but — a hiding.
And here's a recent paper by Nikos Nikiforakis, Jorg Oechssler and Anwar Shah:
We have designed a game in which exploitation can result from the hierarchical relationship between players and, in particular, from the fact that the senior worker has the power to coerce a junior worker into exerting high levels of effort. Using a laboratory experiment, we find that senior workers often attempt to exploit junior workers.
This is not an isolated finding: it's consistent with an earlier paper by Ernst Fehr and colleagues.
The question is: what is the external validity of these laboratory findings?
On the one hand, Marx's critics - following Coase (pdf) and Williamson - might argue that hierarchy increases aggregate output and this benefits workers sufficiently to offset opportunistic exploitation. This, however, runs into the problem that cooperative firms can be more efficient than more hierarchical ones.
On the other hand, though, one might argue that the Nikiforakis, Oechssler and Shah paper actually understates the problem. In lab experiments, the payoffs to both parties are small which means that exploitation is mitigated by a norm of fairness. In the real world, however, the payoffs to exploitation are big enough to overwhelm that norm. This is especially the case to the extent that narcissistic bosses' over-inflated sense of entitlement might mean that they feel entitled to extract big rents.
Now, there was a time when the Labour party had an answer to these problems. In the post-war period, it thought that full employment and strong trades unions would give workers a form of countervailing power to resist the tendency for hierarchical capitalism to generate exploitative rent-seeking. Today, though, it's answers - insofar as it has them at all - seem rather weaker.