The phrase "and it is difficult to determine" is correct and usable in written English.
You can use it when discussing a situation or condition where making a decision or conclusion is challenging.
Example: "The data is inconclusive, and it is difficult to determine the exact cause of the issue."
Alternatives: "and it is hard to ascertain" or "and it is challenging to identify".
Groundwater contamination is a serious problem that is growing in the United States, but its true extent is not known and it is difficult to determine because of the complexities of contaminants, their transformation, and fate in groundwater systems.
These responses are invariably excitatory, and it is difficult to determine whether they are sensory, motor, or proprioceptive in origin.
There are more than 6,000 911 call centers nationwide, and it is difficult to determine how much it will cost to bring them to Phase II.
Truth told, elections are complex social events, and it is difficult to determine (let alone predict) what matters, how much it matters and in what sense it matters.
Therefore, several methodologies are available and it is difficult to determine the most appropriate methodology for specific project within different domains.
The aggressiveness of probe message transmission has a great impact on sensing latency, and it is difficult to determine the optimal aggressive degree of probe message transmission.
The stress state around a moving indenter scratching a coating/substrate system is very complex and it is difficult to determine the stresses which lead to detachment.
Perhaps the guerrillas' most important method of financing, kidnaping has taken on the dimensions of a national industry and it is difficult to determine whether guerrillas or common criminals are responsible for any particular crime.
And it is difficult to determine what might have inspired the state to protect an animal regarded still by the innocent as a monster and by the less innocent as more valuable dead than alive.
