Having functioning and accurate reverse-DNS is required for mail servers, as lots of mailservers reject mail from servers without correct reverse DNS. There's likely other protocols where having functioning reverse DNS is a necessity or strongly advantageous as well.
Along with other limitations such as "we do not support IPv6 floating IPs. All floating IPs are IPv4" and "floating IPs do not support SMTP traffic"
https://www.digitalocean.com/docs/networking/floating-ips/