Verification becomes difficult, when the system gains adoption and processes thousands of TX per second. Then the issue becomes about the trade-off between node resource requirements, and decentralization. Fundamentally, every node having to process/verify every single transaction in the network,is not scalable.
Hence the ongoing debate around whether verification should be less computationally costly than computation (BTC) or whether a system can be architected to successfully scale even when verification == computation (ETH).
