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 …