Thanks for the "Best Answer", it is appreciated!
I'll give you answers based on what it has and currently hasn't done for me and this is based on testing with FTDNA. So, I have tested yDNA and auDNA for myself, mtDNA and auDNA for my parent's and auDNA for two of my Mum's cousins.
My yDNA test has connected me with another Canning and it is a very close match. But we have not been able to connect the two families with genealogy as we most likely connect in Ireland and the records aren't that good in that area. So tantalising!
The mtDNA tests on my Mum and Dad have not produced any "hits" at this time, but could in the future. Fingers crossed.
The auDNA has been more successful. With my cousin's their Grandmother was an unofficial adoptee http://www.wikitree.com/index.php?title=Unknown-204791&public=1
. So that would have been a dead end line, but a close hit (possible 2nd or 3rd Cousin match) means that we have an indication of the family where one or both of her parents may be from.
The auDNA has also been good at confirming what we have on paper is accurate.
Earlier I mentioned the COULD for auDNA and the reason I stressed that was that just because someone doesn't share DNA with you doesn't necessarily mean that they aren't related. I had a match for my Mum's kit that didn't show as a match for her cousin Ed, but when I got the test results for his sister both Mum and Anita did match. This is why I suggested testing you, your husband and your father as a minimum, as we only get a portion of the DNA from each of our parents and they from theirs'.
So, while there may be limitations in the DNA testing presently, as it gains more traction and better coverage it will become more useful for everyone.
With you trying to discover more about your German and Portuguese heritage what auDNA testing may uncover is descendants of siblings for these ancestors who may have taken an alternative route to the US. These people may have knowledge (and family treasures) about the family that you don't.
I apologise for the verbosity of this "answer" and realise that it will probably generate more questions rather than giving you a definitive answer :-)