A challenge in Macro Photography is the depth of field. Generally, the area in focus is so shallow, that large parts of the image appear out of focus. More often than not, macro photos are taken at higher F-stops of F11 - F16 (or till the point where the diffraction of the small aperture starts to become an issue).
Also, the area under focus is so small, that more often than not, there is not enough light for ISO 100/200 photography and photographers increase the light in the scene using a macro-flash, spots,reflectors etc.
While a large f-stop allows maximum light, it is often not enough and additional light sources are necessary. Also, the large f-stop can actually break some macro-shots (they can also make some macro shots).
The 50 mm focal length too is quite short and to achieve the lens's MRR of 1:2, you might find the lens almost touching the subject.
Focusing speed is not always critical. When doing macro-photography, more often than not photographers use Manual Focus because auto-focus can get confused and focus on the wrong area of the subject. This is particularly noticed when photographing objects where contrast is less - for example close-ups of petals, leaves, diamond jewellery etc. Also, if you are considering Focus-Stacking, then manual focus is mandatory.
As a portrait lens, the 50mm f2.5 would be terrific. But then again, the Canon 50mm f1.8 is available at throw-away price too (though it's MRR is close to 1:6). For portraiture work, I would choose the 50mm f1.8 / f1.4 any-day.
If you are serious about macro photography, my recommendation is that you consider a 90mm 1:1 Macro lens. Tamron makes one. Canon also has a similar lens (similar focal length, double the pricing). 90mm gives you sufficient working distance for flowers / jewellery etc. It's also great for portraits and gives you enough working distance so that the model does not end up feeling that the photographer is all over her.
For live insect / fluids / flammable etc. macro work, you will benefit from even higher working distance. Hence macro lenses with even higher focal-length are recommended.