I'd say our disagreement is semantics. I wouldn't say tsuyoi, but I see aikido in terms of skill, thus better or worse. My dojo member would never say sono hito ha tsuyoi ne. they would say umai na. or sugoi. that's just my dojo, not everyone sees it that way.by the way, the people who are tsuyoi at my dojo are rarely umai, because they can't relax and they use their strength instead of the technique.
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.