Magnetic order is prohibited in the 2D Heisenberg isotropic model at finite temperatures by the long-established theorem put forward by Mermin and Wagner. Indeed, the recent studies on true 2D magnets have nothing but confirmed that magnetic anisotropy is pivotal for eliminating this restriction and allowing, for example, the onset of 2D ferromagnetism in a bilayer of chromium germanium telluride, or in a single layer of chromium triiodide (CrI3). These materials happen to be two identical uniaxial systems from the point of view of their magnetic anisotropy. However, the critical behavior of these 2D magnets is still a matter of debate amidst contradictory reports of the critical exponents. By modelling magnetometry experiments on bulk crystals and magneto-optic data of the monolayer we will shed new light on the universality class of these van der Waals magnets.