fpps pays you a fixed rate per share.
Tides pays you based on your share contribution as a percentage of all shares for that block.
For variable hash your payout doesn't become worse, but it does become variable. Of course tides hash payout is variable for fixed hashrates too.
My first ocean experiment I got lucky, 8 hours of hash got me 3x what I was getting daily from braiins once my entire 8 block window was payed. Later over a 3 month window test I got payed more than the previous 3 months with braiins even though fees were lower during the ocean months. I definitely had days and weeks where I got less on ocean than the old fixed rate but over time it was more.