DTTG apparently has its troubles with large databases. My most used DT database is a collection of font specimen, that I like to have at hand for reference. Most files are PDFs, quite a few image files, and an occasional web archive or RTF(D). That amounts to about 1 GB in size, almost all of which is synced to DTTG.
To open the database(s) on the phone takes really long!* That would be a minor issue, if only they would remain open. (This might be an issue with the iPhones memory handling, though.) So, it feels like the app is there in a second, but then takes a minute to open the db.
Then, each sync (that usually copies just a few new files; between some hundred KB and, say, 5 MB at a time) usually takes more than one attempt – DTTG almost always crashes at least once, and usually somewhere near the end of the sync (if the progress bar can be trusted). Also clearing up (or whatever happens, when the progress bar looks like finished, with the name of the last file still showing below) takes longer (much longer!) than the actual copying itself.
Once it is synced and opened, it mostly runs fine, with just a very rare crash on some single file. (That’s still annoying, since re-opening DTTG can take the aforementioned minute, if the db also has to be re-opened!)
One thing I miss is a sync of tags. In DT Pro, I have sorted the font specimen in folders by foundry. To be able to search by classification or appearance, I apply tags to the files (like “grotesque”, “script”, or “geometric”, “lively”). Will tags be supported by DTTG in a future version?
*Tech note: DTTG runs on an iPhone 4S, syncing to DT Pro on a MacBook Pro (2010) over 802.11n.