The phrase "are sparse and often" is correct and usable in written English.
It can be used to describe something that is infrequent or limited in quantity or occurrence.
Example: "The resources available for the project are sparse and often inadequate for our needs."
Alternatives: "are limited and frequently" or "are few and typically".
However, evidences in this setting are sparse and often related to single-center reports.
Currently used in-situ water monitoring techniques are sparse, and often difficult to execute.
Establishment and firm microdata pose many challenges in this dimension, as they are sparse and often unique.
However, due to the prevailing aridity, paleoenvironmental archives within the southwestern Kalahari are sparse and often discontinuous.
Accurate contemporaneous data on the incidence of pre-eclampsia in high-income settings are sparse and often limited by coding inaccuracies.
Most species have not yet been described and even for the minority that are known, data on spatial distributions are sparse and often unreliable.
Among the remaining nine-tenths, the vast majority of whom qualified for it, "knowledge of the right to buy was found to be sparse and often inaccurate".
The digital prints were all the more effective for being sparse and often seeming to blend with the swirling digital backdrop.
In other regions the presence of ammonites that are useful for correlation is sparse, and often these species cover a wide range of time or are not easily correlated to other areas.
The information found online about them was sparse and often lacking major details like the artist's name.
Wildlife in prime areas in Kenya (e.g., Mara) has declined to a third or less of its former (1977) abundance (Ogutu et al. 2011), and there may well be similar downward trends in Tanzania, but data is sparse and often restricted.
