All Epochs are dropped

It really depends on your data and the recording system used. We usually pick quite large values to only capture sudden “spikes” in the signal, which would e.g. throw off ICA. Something like {"eeg": 500e-6} might be a good start. Be sure to look at epochs.plot_drop_log() output to figure out if and which individual channels contribute to rejections.

For flatness, I don’t usually do rejection…

The function mne.preprocessing.annotate_amplitude() might also be of interest to you. It operates on continuous data.

Best wishes,
Richard