Of course it is abour semantics. Different people bring in different experiences in to the translation. I bet that if you asked ten native japanese speakers you would get a lot of different answers to your question.
For me, and the people in my dojo, tsuyoi does not imply that you are stiff or use a lot of muscle in your waza. Katai on the other hand does.
If i were to say that someones aikido was good, i would probably use umai or joozu. Sugoi for me has a very different meaning.
I don't think it is semantics.
If I tell one of my fellow club members or seniors "Waza ga Tsuyoi" or "Aiki ga Tsuyoi", in my mind, I'm telling them that their technique/application of Aiki was very powerful/appropriate, yet almost all will interpret my words as critical.
"Tsuyoi" is connected with "chikara" which is a type of strength that is usually inappropriate in traditional Japanese arts.