What are your thoughts on cryptocurrency such as bitcoin and ethereum?

Um - need a few more specifics on what aspect but a few random thoughts:

Cryptocurrency is so broad with dozens of coins these days. But my overall feeling is only Bitcoin is really 'viable' with 2nd place either Etherum or Litecoin. Litecoin is just a clone but been around longer while there is some interesting innovations for Etherum but seems a bit too bleeding edge sometimes.

At least from the viable ecosystem standpoint - Bitcoin definitely has a firm foot - exchanges, tech stacks, it is all there. but still is this going to be long long term viable? harder to say since I still have yet to see an application on blockchain that really is useful/only possible because of blockchain - Etherum is headed in that direction but as I said sometimes so complex and potential for unintended consequence so make me worried.