The primary downside is expense. The biohaven islands shown above cost $35 per square foot. So even a relatively small 7' x 5' island is 35 ft square, which would be $1225. That's before adding plants!
So, I've been thinking of experimenting with 'something else' to float and plant on. Any of our plant biologists have some hard earned experience to share?

"Politics": derived from 'poly' meaning many, and 'tics' meaning 'blood sucking parasites'.