Dear Sacred, it can only be positive!
On the aspect of medieval authenticity - I personally do not care much for Salva Nos, even though its sound is probably more authentic than that of Illumination. I find the sound of that album is rather...dull. Most of the material is acapella and that kind of bores me out, in general.
Nevertheless, I started out as a fan by looking at Worldes Blysse at our (deceased) local Virgin store and wondering what it sounded like. One day I decided to have a listen and as we all know that album is quite medieval.
Even so, my love for the Baebes only grew and grew as Undrentide's bashfulness blew me away - I loved the album much to the contrary of the general fanbase's opinion!

(Looking back on it , or rather hearing back on it, I can now understand why! LOL!)
The Rose didn't captivate me as much, it was a return to the more slower ballads and songs which always somehow seemed to last too long (still, I do like Blow Northern Wind...it is nice and calming. Almost entrancing.).
Then came Mirabilis and that for me, was the best album until Illumination came out! These two, although perhaps not quite authentic to the medieval age, do work as albums. I see the Baebes as a kind of cross-over artist more - they use whatever influence they get and do not conform to general opinion of how to create a certain song, or a certain mood. They follow their own mind and ideas which I applaud!!

It makes them unique in their own right. As I listened to WB I had that same feeling of uniqueness - I had never heard anything like it!
Illumination is the culmination of years of experience combined with fresh 'Maiden' blood and a drive to experiment. In previous albums they stuck with mainly European sources, but it is so wonderful to see the Baebes now extend their vision across borders - I would love to hear a song in Japanese once!! Or Chinese!! Or anything Asian - I'm sure it would rock! (seeing the effect Mad Song has on me)
And, all in all Simon - it's fine that you do not like the album as much (yet) - We all have albums that we don't like as much as another album. it is what makes us unique - like the Baebes themselves
So, just relax, let the songs work their way into your system ... and see what happens!
Love,
Phantas