It's hard to say. For example the Google's paper talks about some rare (once an hour) strong errors. Are they fundamental or have some easy fix? We don't know.

It's hard to say. For example the Google's paper talks about some rare (once an hour) strong errors. Are they fundamental or have some easy fix? We don't know. One obvious problem is cooling. We can't cool million qubits in a single fridge, so we will need to split them between fridges and communicate. Also, the wiring is really complicated already and hard to scale (one reason IBM has heavy hex is to have more space). Another problem is connectivity. For transmon qubits connectivity is fixed. Applying gate to two far qubits requires a lot of swaps, which is expensive. This is less of a problem for ions or cold atoms because they can couple any two qubits; but they likely wouldn't be able to for large amount of qubits. Another thing is classical control, because the classical data needs to be processed at high speed. Some companies develop specialized hardware for that. None of these is necessarily fundamental, but these problems need to be solved, in addition to usual scaling (it is hard to manufacture these devices and it becomes harder with each qubit).