neither … nor …
This does not imply that …
… begs the question …
One question is whether …
It is questionable or doubtful whether …
It is by no means clear that …
This raises the question whether …
We can question whether in fact …
While it may well be true that …, …
While it may well be valid that …, …
While we cannot deny that …, we …
… inconclusive evidence
… insufficient proof
… discouraging results
… unsatisfactory results
to outline the case against …
it is important to consider …
it is important to take account of …
it is important not to overlook that …
the evidence for … is sketchy at best
… fails to take … into account …
… is or are frequently overlooked in discussion of …
It is frequently overlooked that …
Although … may play an important role, it is clearly not the only factor.
The problem, however, is that …
It does not follow that …
One difficulty is …
a or the lack of …
it does not easily explain …
Difficulties arise in …
… poses particular problems in cases where …
the problems identified also apply to …
A further problem is …
A potential problem is …
This is further complicated by …
A or the flaw in this argument is that …
A more serious failing is that …
A or one major weakness of … is …
A serious drawback with or of this approach is …
One of the prime failings of this theory or explanation etc. is …
The or one problem with this explanation or approach etc. is that …
A serious question remains concerning …
Serious doubts have been raised about …
However, there is also evidence of or for …
However, there is also evidence to suggest that …
It is probable, therefore, that … reflects or represents … rather than …
It is by no means the case that …
… the present study indicates that this is not the case.
It is frequently claimed that …; however, …
Although many authors would agree that …, …
From a … standpoint, however, …
Crucially, however, …
However, it would be rash to conclude from this that …
However, we cannot be certain that or whether …
The relevant or important factor here is not …, rather …
However, none of the explanations accounts for …
The explanation simply does not hold water.
An alternative explanation would be …
a more likely interpretation is that …