Well, I've had BTC transactions drop out of the mempool (so, they were not executed, but you don't know that immediately, you only know days later), and transactions that worked took between a few minutes and more than half a day. The fees ranged from virtually nothing to around 50 USD. I certainly would not call that predictable and stable. I mean, you don't even know when the next block will arrive.
I would be suprised if a valid normal transaction with 50 USD would drop out of a mempool. What was the transaction weight? If you send the transaction data, I would be interested to look at it.
I'm usually overpaying (20 satoshis/byte, usually under $1 for a $2000 transaction), and I always get into the first block, but I never pay $50 fee.
