in a perfect world arbitrage betting indeed has no risk since you are betting all the possible outcomes and benefiting from the different odds in different sites
BUT almost all sites are against arbitrage bettors ( except few asian ones )  which means sometimes the books may void your winning bets so you end up losing

there used to be a similar bitcoin service called satoshi signals and it used to scan the books to find arbs , it lasted a month or 2 since it stopped due to lack of interest
at that time most of their arbs had a site called powerbet ( which scammed and closed long time ago ) and they didn't tolerate the arbs so they ended scamming some people or at least limiting them to bet only small amounts

in short arbitrage does exist , but it's really not worth it with bitcoin sites ( actually even fiat sites don't tolerate arbitrage bettors )