Most references I can find about the Macuahuitl's effectiveness against Spanish forces are about their use against Spanish Horses, but nothing about their use against Spanish infantry. Would it be extremely difficult for an Aztec soldier to kill a Spaniard in steel armor with a Macuahuitl? If so, how did the Spanish infantry suffer such high casualties during the war against the Aztecs?
They are described as effective by conquistadores, who gave descriptions such as
Their weapons were sharp blades of flint, set into opposite sides of a club, and this weapon was so fierce that they claimed that with one blow they could chop off a horse’s head, cutting right through the neck
and
they can split a man in two with a single blow, provided this is the first one, since all further blows are useless, such is the sharpness of this weapon and its fragility.
(Quotes from Cervera Obregón (2006).)
However, there is no reason to think that these apply to armoured targets. How effective might it have been against armour? Sharper than a steel sword (as long as the obsidian blades were still sufficiently intact), and probably of weight similar to swords. Thus, against plate and mail, ineffective. Possibly effective against textile armours (such as the cotton and/or kapok armours used in Mesoamerica), since sharpness is useful for cutting through textile armours.
Note that many reproductions are quite heavy, and some estimates of their weight are quite high. E.g., the reproduction described in Cervera Obregón (2006) was 1.5kg, and the Wikipedia macuahuitl page gives 2-3kg (without referencing any source for this weight). However, if we consider common weights for war clubs, one-handed war clubs are usually much lighter than this (and most macuahuitls shown in artwork are used one-handed), and two-handed clubs average about 1.5kg. (From the data given by De Vere Bailey (1947), one-handed clubs over 70cm in length vary in weight from under 400g to about 1.2kg; the average is 0.87kg. The two-handed clubs vary from 0.86kg to a lone monster of 3.37kg, averaging 1.57kg. Removing the 3.37kg club from the calculation, the average is 1.32kg.) Sword-clubs (i.e., flat, blade-shaped clubs with a narrow striking edge but without the obsidian blades) are often lighter. Clubs are often lighter than swords because (a) more of their weight is near the tip, and (b) they are often unsuited for thrusting, and must be swung to be effective. Putting these two points together, clubs must be lighter to be at an effective fighting weight.
Thus, a macuahuitl should not be thought of as a heavy anti-armour mace.
So, how then might Spanish soldiers have died? First, they weren't wearing complete armour, and a macuahuitl could still be effective. Second, there were plenty of spears and arrow, and stones (spears and arrows are perhaps more likely to find gaps such as the face). Third, disease.
Reference:
Marco Antonio Cervera Obregón (2006), "The macuahuitl: an innovative weapon of the Late Post-Classic in Mesoamerica", Arms & Armour 3:2, 127-148, https://doi.org/10.1179/174962606X136892
De Vere Bailey, B. (1947). NOTES ON OCEANIAN WAR CLUBS. The Journal of the Polynesian Society 56(1), 3-17. http://www.jstor.org/stable/20703072
An example of a sword-club of somewhat macuahuitl-like appearance (without blades): https://britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=501130&partId=1
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