Off the top of my head, without reviewing purchased scenes or overthinking/analysing, just my first thoughts, personal likes and dislikes:
Gonzo=====

book many good girls local and international

steady repeat bookings for good girls

variety of girls including curvier models e.g. Taylee

consistently good lighting and camera

good MUA

scene prices good value relative to other PB studios

no longer any variety in scene locations

churn out too many cookie-cutter scenes

past production hiatus when directors out of town
Gio/GL (Prague studios)
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book many good girls local and international

steady repeat bookings for good girls

consistently good lighting and camera

good MUA

1:1 introduction scenes (personal preference)

rarely any variety in scene locations

churn out too many cookie-cutter scenes

heavy bias towards models wearing lingerie (probably sells well, just my personal dislike)

books quite a few tattooed/alt/enhanced models (good for those girls, just not my cup of tea)

but won't book curvier models e.g. Taylee (personal preference)
NRX===

Scout many cute new girls

Fresh ideas for scene scripts

variety in scene locations

Patchy lighting and camera quality

sometimes the theme/script ideas overtake and distract from the porn action

Some scenes are dark and use "creative" coloured lighting (personal reference)
VK===

Cute girls, some new

Variety in scene locations, including outdoor

Consistent camera and lighting quality, experienced producer

None really (but I just enjoy firstanalquest-alike scenes)
N&F===

Nice small family business studio vibe

Fresh POV and ideas for scripts

Introduce some nice less-known models e.g. Lenna Ross

variety of girls including curvier models e.g. Taylee

Inexperience shows in some lighting and camera cf. Gonzo/Gio (but improving)
Andy Casanova=============

Introduces some cute new Italian girls

Porn Academy/feeder studio for best girls to graduate to Giorgio's productions

Fresh POV and ideas for comedy scripts e.g Italian TV gameshow wheel-of-fortune thingy

too much teeny/schoolie/almost incest themes (personal preference)

too much yucky male rimming and toe sucking distracts focus from the girl (personal preference)