I think difficult to manage means the shear volume of posts which had to be moderated. When I used to be a moderator on another site even with 10 of us online at peak times during the day it could be a nightmare.
The message is probably means the old one wasn't that efficient it only takes a certain number of SQL requests to a database like MySQL and the entire site can come down. That said the site I used to be moderate was far far far busier than HJ and managed fine with the LAMP configuration but used VBulliten.
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