When people say it's deflationary, they mean "in the long run". Bitcoin has a fixed, predetermined inflation schedule which approaches zero over time. So, cons

When people say it's deflationary, they mean "in the long run". Bitcoin has a fixed, predetermined inflation schedule which approaches zero over time. So, considering the inflation schedule alone, it's "eventually" deflationary. When you start to account for other things like lost and unrecoverable coins, it becomes severely deflationary and much sooner. Things like grandpa taking his encrypted wallet to the grave or in the case of bitcoin BTC, small coins that can't be moved because the fee is higher than the coin's value. At the time of writing this, coins worth less than ~ $6.15 (USD) aren't able to be spent*. That means if you bought a $4 coffee with BTC and paid with a tenner (a coin [input] worth $10) , you'd lose the change. It would remain unspendable until the network starts unclogging. * By "arent able to be spent" I mean, reasonable fee estimations for getting the transaction into a block within an amount of time that you can be sure the transaction won't get dropped from the mempool. I used https://bitcoiner.live https://bitcoiner.live to estimate the fee which was the most generous of the three I looked at.