The phrase "are very difficult to verify" is correct and usable in written English.
You can use it when discussing information, claims, or data that are challenging to confirm or validate.
Example: "The results of the experiment are very difficult to verify due to the lack of consistent data."
Alternatives: "are hard to confirm" or "are challenging to validate."
And because the area is off-limits to foreign journalists, it has been very difficult to verify the competing claims.
A cease-fire would also be very difficult to verify, he said, and could easily be broken.
According to Pelizzaro, Litvinenko "was giving Scaramella lots of information about Russian and criminal infiltrations in Italy, but most of it was very difficult to verify and crosscheck.
Without access to his tax records, it is very difficult to verify the claims he has made about his finances, and it's impossible to ascertain how much he has paid in taxes.
"Although almost every theoretical physicist agrees with my prediction that a black hole should glow like a hot body, it would be very difficult to verify experimentally because the temperature of a macroscopic black hole is so low," he added.
However, experts have cautioned that it is very difficult to verify or assess such claims.
Note that the equation which one has as a result is very difficult to verify.
However, it's very difficult to verify the exact number of people executed each year for bringing drugs into the country, as the use of the death penalty in Vietnam is classified as a state secret.
But without participation from the complainant, "it is very difficult to verify allegations," the school said.
Aside from statistical approaches, it is very difficult to verify the existence of unpublished studies or the presence of excessive selective reporting; for alternative methodologies including surveys and experimental approaches see refs. [2], [15] [17].
