Who said betting on a single match is reliable? As we say that the money you deposited in a casino is highly likely to be lost, so is the money you deposited on sport betting sites, regardless of the odds that you go for. Only what people can say is that parlay is riskier, but it has its own advantages.
No pattern of bet can be reliable as far as these games are unpredictable, the same way a parlay bet can spoil your bet slip that's how a single bet game can do too. What is there about single bets is that they have higher winning rates when played much often than a parlay bet would have for the gambler. For you to be playing single bets you must have a huge staking power to boast your potential win that's why many gambler that don't have the funds don't go regularly with it.