Makes you solely dependent on Google. That's not really a problem, but as we've seen many times on HN and elsewhere, there's always the random chance that Google's automated systems can decide that your account meets some unknown criterion to be locked, deleted, or otherwise preventing you access.

It's rather unlikely, but so is the chance of your house burning down.

That's generally the reason people recommend the 3-2-1 rule. By having multiple separate backup solutions, you're hedging your bets. But as is commonly the case in computing, you have to play the convenience vs security/reliability game here and decide where the line between your time and convenience lies against the reliability of your backups and what risks your willing to take.