www.languagecourse.net vocabulary trainer VT and www.lingvist.com let’s you learn mixed / random vocabulary by throwing words from different “categories / topics / groups”.
Lingvist uses the “fill in the word into the blank” technique but shows a complete sentence in the target language and reads it out load; you can can additionally click the “Arrow up symbol” on the top right side and the L1 source/base sentence will be displayed above the grey panel (the single base/source word is below the panel).
In comparison DuoLingo:
There are topics (skills in a tree, e.g animals, food) with up to ~10 lessons.
One lesson usually contains up to ~7 words and has multiple questions (can either be single word, quite short, normal or longer sentences with mixed typed of exercises, e.g pictures, listen audio, translation, enter a single word, etc.).
So yes, the “problem” with learning words all from the same categoriy (topic) even applies to DuoLingo.
I tried to use both (DuoLingo and Memrise) in parallel for 1 year, either focusing on prelearing/re-learning single words on Memrise with MartinPen’s “PT BR DuoLingo” couse, seeing some “real world practical phrases / sentences” with the Memrise PT 1-7 courses, and then going back each day to DuoLingo for all the Portuguese grammar skills, having mixed/random sentences for the “vocabulary skills” on reviews…
Mondly www.mondlylanguages.com uses the same tree topic concepts, whereas the topics may be more theme-oriented / practically divided (e.g restaurant, airport, etc.) which DuoLingo does not provide yet.
There are many peple on tehd DuoLingo discussion forum who have great experiences about retaining the learned vocabulary from Lingvist, as it seems. They wrote it to me personally or in general “Lingvist” threads. It was suggested multiple times.
Currently I registered for for Portuguese-English reverse course, where I can see the PT-sentence (top over the panel), but only get the reverse EN translation and English audio 
I hope they add the forward course EN-PT next year, like they do with Spanish, French, etc.!
What did you try? What worked best for you?