I previously regularly imported my last.fm data manually. But as I’ve heard about the new possibily to connect with your last.fm account, I purged all my listens and used this function instead.
But there is one problem: If past scrobbles are added (e.g. because they were cached, or you used openscrobbler.com with standard settings) but other scrobbles have already been added that were synced with listenbrainz, those past scrobbles won’t be added to listenbrainz.
My example: Yesterday I listened to music with my music player foobar2000 until 10:02 pm (listening to Garoad’s VA-11 HALL-A - Second Round). About one hour later I manually scrobbled the first disc of Venom’s In Nomine Satanas - The Neat Anthology (40 Years In Sodom) and Depravation’s II:MALEDICTVM. (When manually scrobbling I don’t really care that much about the accuracy of the timestamp.) You can notice the difference between my last.fm scrobbles and my listenbrainz listens. Listenbrainz only synced those Venom and Deprevation scrobbles after the last Garoad scrobble.
I know my problem is very niche, but I think this can also be a problem with cached scrobbles.
Edit: Sorry, I’m stupid and now I’ve deleted my listenbrainz listens so I can go back to manually importing last.fm history. So my example is less helpful now, but hopefully my explanation is enough.