There are many problems with bitcoin in particular and blockchain in general.
Too many, in fact, for me to waste your time reading them all.
(Image courtesy Tony Arcieri)
I'm not interested in the economics or politics of bitcoin/cryptocurrency. I'm interested in the cryptography engineering.
My biggest beef with Bitcoin is that, a lot of Bitcoin users believe their payments are private or anonymous, but they aren't. You want Zcash instead.
My second-biggest beef with Bitcoin/Blockchain is that it's a magnet for charlatans and fraudsters. I choose to spend my time and energy in other areas.