Disadvantages of a looper
Of course, a looper needs a lot of space on the pedalboard.
If you use a small pedalboard and the looper needs so much space that there is only room for three or four other pedals, of which some stay constantly in the effect chain (like booster, delay or reverb), the question if you need a looper is answered by itself.
Besides, keep in mind, that a looper also needs a power supply.
Especially if it provides midi and other functions, a looper is absolutely power-hungry.
Though you should also take an appropriate power supply into consideration while you are planning your pedalboard.
Impedance mismatches as well as humming and problems with your buffer or fuzz pedal might stem from the use of a looper.
In this case, the elimination of the source of error requires significantly more effort than activating the pedals separately without a looper.
Concerning the wiring you should be aware that the cable paths form a looper to a pedal and back are much longer than the paths of cables between two pedals.
By activating two or even three loops at the same time, the resulting extended cable paths might worthen the signal.
And, many cables offer many sources of error.
Special Tipp: The KISS principle (Keep it simple, Stupid) is really helpful while you plan your pedalboard.
And the fact, that many pedals provide true bypass it is sometimes reasonable to dispense with a looper and rather use the pedals in a row with a good buffer at the beginning of the effect chain.
Advantages of a looper
One of the main advantages of a looper is, that it allows to remove the idle pedals from the effect chain.
Most guitarists with large pedalboards do not play either without any effects nor with all pedals switched on at the same time. Therefore, a looper makes definitively sense when you use a huge pedalboard.
And if you have many pedals on your board and want to switch quickly between clean sounds with effects and overdriven sounds, a looper might is advantageous.
Nu Metal-guitarists for example benefit from using a looper for songs with only few transitions and abrupt alternations to switch between different styles of sound.
By the way, for those who use several midi controlled multi effect pedals a looper can also be quite interesting because it is able to send midi commands and provides many different presets.