Illegal repeaters/boosters abound. They can block mobile signal reaching your neighbours, and they can even bring down phone towers cutting signal to thousands of users. Penalties for operating illegal repeaters range from fines to jail terms.
Cel-Fi products differentiate themselves from illegal products by being in the public forum; the product is supported with product reviews and media releases etc. You will find the illegal companies do not do this.
Find below a letter from the Australian Communications and Media Authority (ACMA) listing the companies who sell illegal products: Mobile Repeaters Australia, Signal Boosters Australia, Amplify Australia. They operate virtual premises and phone numbers that look like they are based in Australia or New Zealand, in reality they are based overseas making it very hard to shut them down. Ebay has also recently banned all illegal repeaters from sale on their platform.