First, I give it 5 Stars to appreciate a RPGM H games with big effort, I think the game sould be 4.3 for my rating.
This game is RPG first, H-Game second. a solid mechanic and manage to avoid common H-Games mistakes.
Actual keys for fast travel. most RPGM H-Games are not intuitive (due to how RPGM by default). IslandSAGA has neat navigation and also speed-up combat animation for smooth no-time wasting gameplay
it's not a walking simulator. say goodbye to usual 30tiles long in 1 map
H-Scenes is 2D-animated. it's satisfying and feels like a new experience, AFAIK not much H-Games handle H-scenes Animation like IslandSAGA
But of course the game has it's flaw, even compared with Kamichichi last project, Apostle.
Compared to Apostle, the casts is a downgrade. Protagonist is silent and has no personality, you can still give personality like Apostle did and still open to choices.
The female casts also a downgrade IMO. in IslandSAGA all the females already "sexually interested" with the MC from the get go (even Sakura who seems not interested story-wise). the build-up is weird. in Apostle the build up is more organic and believeable, you start with peeking scene, accident scene, until you get Sex Scene.
No niche small/normal sized boobs cast. I know the circle is about big boobs girl, but at least Apostle insert Kenshin as niche sized boobs.
there are fewer H-Scenes overall, I think this is the cost of animated scene. no scene other than 4 female casts (as far as I'm playing now) and mostly have same formula (quest accident, first sex, fetish sex scene). Apostle has scene outside of party member, even from story-quest. IMO lack variation.
Most girl already have same motivation from the get go, so again lack variety of response (in Apostle, there are childhood friend, Tsundere, Nerd, clueless tomboy, mature woman,Rich girl, and some girls from story that still contribute to H-Scenes)
That aside, Island SAGA is a high quality RPGM H-Games, despite it's flaw,the game is one level higher than common RPGM game you find here