Valks always have use. The reason th8s don't use them is their ai is a bit trciky and they can be unpredictable at times.
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And...thats why TH8s dont use her. Yeah, 8 housing space is a lot, so it could be changed. Remember that valkyries cost dark elixir and are unlocked in late stages, so why should they resemble giants?
Seriously, only differences are that she does splash damage, shes quicker and she doesnt attack defences first.
Valks are good as they are imo. I have seen many good 3 stars govalo, gowiva on th8s with lvl1/2 valks. They will become op with any buff. U just have to be good to know when and how to use them. I cant say about higher lvls yet but afaik they r good too.