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It would be useful to know a bit about what part of the world you are making enquiries for as the UASB has been particularly effective in tropical locations where temperatures are warmer and relatively stable.

In general the greatest risk with a UASB is the possibility of washout of the biomass if the HRT (hydraulic retention time) is too low. To maintain the population of useful microorganisms and anaerobes a larger reactor volume or HRT may be required which can lead to higher capital costs.

The UASB has a long start up time and recovery from stressed conditions can also take many days, making control of the reactor important. In particular control of factors that will affect operation such as temperature, pH and influx of any toxic substances in the feed stream. Operation of the UASB requires trained and experienced operators. Typically it has been used for industrial wastewater treatment and seen as less effective on dilute municipal feeds but more recent design improvements have seen the use of UASB in municipal sewage treatment in countries such as Brazil.

The following paper gives quite a good review of the UASB: Bal AS, Dhagat NN (April 2001). "Upflow anaerobic sludge blanket reactor—a review". Indian J Environ Health 43 (2): 1–82. https://www.ncbi.nlm.nih.gov/pubmed/12397675

Regards Harriette