The limitations of current knowledge
At present, little is known about …
Very little is known about …
There is as yet no clear evidence of or for …
Little attention has been paid to …
There has been as yet no systematic examination of …
Research in this area has been limited to …
… research has concentrated on …
Very little has been written on or about …
… is or are frequently overlooked in discussions of …
… has rarely been done before
… is not yet clearly or completely understood
Our understanding to date has been limited to …
… has received very little attention in the literature
there is as yet or at present no consensus on …
there is at present no general agreement on the causes of etc. …
no satisfactory account or explanation of … has been given or provided
this diversity of approaches or methods reflects …
attempts to … have so far proved unsuccessful
The initial claims that … were unduly positive
evaluation is hindered or made more difficult by …
It has not been demonstrated unequivocally or conclusively that …
there are difficulties with all or a number of these explanations
at least in its or their current or present form
The question remains, however, …
a or one persistent problem in … is …
X and X’s results posed a number of questions which we have so far been unable to answer
Although much important work has been carried out on …, a number of questions remain.
Although considerable progress has been made in …, many important issues remain unexplored or unresolved.
Although much has been learned about … over the past … years, a number of fundamental questions remain.