The problem seems to the definition of “Boutique” hotels. I looked at the Wikipedia definition and it has nothing to do with my idea of a Boutique hotel which implies small (nowhere near 100 rooms) – somewhere intimate.
My experience is limited to a few in Spain, in Britain and a couple in other countries.
The ones in Spain have been from 8 rooms to about 45 and their star rating is usually 3 or 4 – this is because the grading system in Spain demands better levels of service with more stars which small hotels plainly cannot provide.
This does not mean that small hotels are any less attractive or you are any less likely to get good service. They can be majestic (ex-residences of the nobility) or quaint (I went to one near Zamora which was a converted old flour mill – quite delightful).
Then again Spain has the marvellous “Paradores” which tend to be restored palaces, castles, monasteries and so on. These tend to be bigger and offer a higher standard of service. The prices are not ridiculously expensive.
I have experienced two (a) Hondarrabia near San Sebastian(Basque Country) on the border with France and (b) Olite( a fairy tale castle in Navarra) south of Pamplona. The prices ranged from €90 to €120 per night single room.