I guess excellent service and transparency builds reputation. It is true thay it takes time and patience to build reputation. A random accusation cannot destroy a reputable casino. Loyal platform users will eventually defend a casino if they know based on their experience that they are truthful of their service. But it is true with just one wrong move, year long reputation can be damaged or be gone in a short span of time.
A random accusation without substantiating any proof must be taken lightly like a drop in the ocean. Many would attempt to destroy the reputation of a gambling website but their long standing and history would prove otherwise.
Unfortunately, with the existence of lots of fake reviews, there are gambling websites that contain positive reviews but mostly from paid users. One of the strongest evidence of genuineness is not the reviews per se, but the time that they are operating in the online-gambling platform.