Sutra:
12. maRRaiyavai iraNdukkum asishtaparikrahamum apUrththiyum uNdu.

Meaning:
The two vyApaka manthras, DvAdasAkshari and Shadakshari have some drawbacks compared to the AshtAkshari vyApaka manthra: they are supported by kudrushtis and they are also incomplete in some respects.

Sri PBA Swami's Sarartha Deepikai:
Why is the Narayana manthra considered higher than the Vasudeva manthra and the Vishnu manthra, even though all three are vyApaka manthras?
Asishta parikraham means they are supported by kudrushtis who claim that the Supreme Lord is "nirvisesha cinmAtram". ApUrthi means that they do not explain everything that they seek out to explain and only talk about one part of it.

Why do kudrushtis support the Vasudeva and Vishnu manthras?
They claim that the Lord has no special kalyANa guNas and that He does not have a special divine form. They only consider His form as one that pervades everything and nothing more. The Vishnu and Vasudeva manthras speak mainly of His all pervading form and not His guNas or His other forms. As such the kudrushtis support these manthras to a great extent.

So what if kudrushtis support these manthras? Why do we have to then consider them lower? Would we stop eating a food item just because it is being eaten by these people?
To answer, this Pillai Lokacharya also points out that these manthras are somewhat incomplete.

It is not sufficient that the manthra explains that He is all pervading. It should also explain what is being pervaded by Him and what is the effect of such pervasiveness. The nature and guNas of He who pervades all, has to be shown. The Vishnu manthra does not do all of this. In the case of Vasudeva manthra, it shows that He pervades everything by being present in the heart (antharyAmi) of everything ("vasudIdi vAsu:"). But it does not speak of who/what is being pervaded. It speaks of His guNas only by the use of another word ("bagavate"). It does not explain why He pervades (that is, the fruit of the pervasiveness). Therefore, the DvAdasAkshari and Shadakshari manthras are incomplete.

Such incompleteness does not qualify the ThiruvashtAkshara mahA manthra. The nAra part explains the nature of who/what is being pervaded. The nArAyaNa part explains that He pervades all ("antharyAmithvam") and that He supports all ("thAragathvam"). The ayana part explains that He is the means ("upAyam") and the end ("upEyam"), for all. Thus, this manthra is complete as it explains what is being pervaded, how it is being pervaded, the effect of the pervasiveness, and the nature of He who pervades all.