I would also add the cons of no lily of the valleys spawning (although I have more than I want now so would gladly give them away)Getting 5 stars will be hard, if not impossible with only 8 villagers.
With my second island, I only have 8 villagers. The two things a that stand out most are
Pro: 8 villagers makes the town feel less cramped. I love it.
Con: You cannot kick villagers out with Amiibos unless you have 10 villagers. This means you will have to patiently wait for villagers to move out if you plan on using an amiibo.
And no golden watering can DIY (but sora/roxas doesn't like gold tools so not a huge issue I suppose)