How well can we tell whether a globular cluster will survive the Galaxy's tidal forces? This is conceptually easy to do if we know the cluster's total mass, mass structure and space motion parameters. This information is used in models that predict the probability of disruption due to tidal stripping, disc and bulge shocking. But just how accurate is the information that goes into these models and, therefore, how reliable are their predictions? To understand the virtues and weaknesses of these models, we have studied in detail three globular clusters (NGC 6397, NGC 6712, NGC 6218) whose predicted interaction with the galaxy is very different. We have used deep HST and VLT data to measure the luminosity function of stars throughout the clusters in order to derive a solid global mass function, which is the best tell-tale of the strength and extent of tidal stripping operated by the Galaxy. We indeed find that the global mass functions of the three clusters are different, but not in the way predicted by the models. [abridged]