By definition Max Clicks bidding will buy the lowest cost clicks possible. This effectively means bottom feeding for search queries that others (using smart bidding) don’t bid or bid very low on. Importantly two different users searching for exactly the same query are not equal… Google understands the difference in intent based on various user characteristics and recent search history.
And of course those “max clicks” queries are far less likely to convert or generate a high quality/value lead/sale if they do.
So while it’s not necessarily “junk” it’s lower relevance search clicks.
Yes, you’re usually better off using manual CPC so you can at least adjust the bids on a per keyword basis noting you are generally using one “max” bid for all matching queries so this really only makes sense with exact match keywords these days.
Even with low projected conversions, like a half dozen a month, automated bidding will usually perform best.