The RVW tuba concerto isn't in Bb major;it is in predominantly Fm, initial key signature four flats with strong modal influences. The second movement is in two sharps.
If you have a version in Bb, then that isn't the VW. It's something else, an arrangement or simplification; AFAIK the only arrangement approved by the copyright owners OUP is that done of the accompaniment by Denis Wick, in arranging the orchestral parts for wind band. The solo part was not changed.
I'm also going to have to disagree with you on 'difficulty being an opinon of the player'. Opinions need time and experience, and there are what might be called industry standards, that are created by a wealth of experience of a lot of performers. In this way one can meaningfully say that the Effie Suite is harder than Bill Bell's 'The Spartan'.
What a young player considers 'difficult' is not necessarily a reflection of actual difficulty, technical challenge, or whatever one wishes to call it. Experience will usually show that 'difficulty' is not a useful criterion. One of the 'hardest' pieces for a solo tubist is 'O Isis und Osiris', from Mozart's Magic Flute,in any arrangement, any key, yet it has nothing more than minims and crotchets! And that's not just my opinion, but that of many players. It's easy to be seduced by a gazillion semiquavers, and to hide behind demisemiquaver triplets, but to convincingly play a simple aria is another thing.....
I'm not saying difficulty doesn't exist, and there's another thread on this forum discussing difficult parts, but that it is a concept to be considered thoroughly and in depth.
Thanks for clarifying the arrangement of the Vivaldi: I do not know the Ostrander arrangement and will look out for it. I have a book of rhythm studies by him, great fun.
Sue