Yes, other Empires lasted (much) longer than Sidious', but they were never able to conquer the Republic. Sidious did. That's the point of the Rule of Two. We can't say the Rule of Two is weak because other Empires couldn't topple the Republic and the Jedi. Only the Bane dynasty could.
Throughout Galactic History we see Sith Empires rise to challenge the Republic but fail miserably. The Bane dynasty and Krayt's Empire are the only ones to ever truly conquer the galaxy.
The Sith Empire held control of a good portion of the galaxy for a long time. The Galactic Empire held the entire galaxy for a short period of time.
Its a feat, but not a worthwhile one imo they hardly conquered the entire galaxy anyway to be more fair, and they didnt control all of the ex republic territories, but thats ok.
I dont see the bane dynasty as weak, at all, but its weaker in this, point, it was designed for survival and anonimaty, and to acomplish a specific task but thats it, other then that it failed where all the sith had failed before in a way, at least the older empires were more stable, were designed to have ruling mechanism in mind that enable the sith order to be in power, even with the infighting. the GE didnt have this, i think the Bane dynasty didnt forthought ruling an empire.
They were weaker in this yes, the only thing they did was to adapt to a situation if you think about it, because they werent strong enough to take on the Republic/ jedi head on, by conventional means.
While sith empires of old were capable of this and more see sacking of coruscant, plenty of times if i recall.