I believe Accor's forte is in their mid-level and "economy" brand portfolios. Thanks to Mercure's flexibility, Accor managed to get unrivalled coverage across Europe, while the new rooms and other enhancements at "red" ibises makes them viable options at quite attractive price points. The cream of the crop is obviously the ibis Styles brand, where a lot of new, often purpose-built, hotels offer both style and great rates, coupled with breakfast included, trumping the less exciting Hamptons and HIExpresses. I love the fact that Accors is pushing this brand actively to franchisees, as the results are attractive properties in otherwise underserved locations.
On the difference between Novotel and Mercure - Mercure was originally intended for conversions, i.e. independent properties and ones poached from other chains, where the operator could acquire an Accor brand with minimal investment and not have to fret about meeting some very strict requirements (e.g. room size in historic properties, or particular fixtures / decor). Novotel, on the other hand, was Accor's original brand being the innovative one in Europe that had entirely standardized purpose-built properties, down to sharing the same L-shaped footprint with an outdoor pool. By now, the Novotels are much more diverse in their footprints, but still the expectation is that Novotels are always purpose-built or at least extensively refurbished (e.g. office building conversions).
The Mercure brand suffers a lot from its resulting inconsistent character though, and I am most puzzled why Accor did not spend more effort on either migrating "better" properties out of the brand, or separating the older ones into a "lower rung" brand (I hope this would be what "Tribe" was about, but it seems they are 25hours / MAMA Shelter without the need to share profits with other brand shareholders). The "new" Mercures, mostly directly-operated properties, are very nice and often above and beyond the current standard for Novotels. It is a shame they share the plaque with so many properties nowhere near their standard, which provides for so many disappointed and/or wary guests.
My experience with Starriott properties in the "limited service" segment (Moxy, AC, Courtyard, Four Points, Aloft) is that they tend to be priced more in line with Mercures, Novotels or Holiday Inns than ibis hotels.
When it comes to upscale / premium brands, Pullman and MGallery are my faves as well. Both can be mixed-bag, but each hotel in both chains (after a number were shed in Europe) tends to provide a memorable and comfortable experience, at the same time often being very competitive price-wise. It does seem Accor has done a surprisingly good job of realigning properties across chains, with small puzzling exceptions (Pullman Aachen, MGallery Milan), and I am happy to see that MGallery was once again promoted to a standalone brand.
The new and refurbished Sofitels do tend to provide far more charm, elegance and character than their counterparts from competing chains (Sheraton, Westin, Marriott, Interconti, Radisson), although I do tend to view them as overpriced vs so many great MGalleries, Pullmans or even Novotels and Mercures.