No one got bumped when we were there and neither ferry we were on was full. There weren’t that many residents traveling those dates. I suspect if you buy your tickets online ahead of time you’ll be fine.
We did see people in line for tickets waiting for a bit and one person told others she knew to, “Just get them online. It’s quicker.” Ferries do sell out - online will say, “Sold Out.” Locals told us they keep some available for last minute, so perhaps that’s one way Hornblower has improved things - reserving slots until the end to avoid having to bump others.
Vieques residents were the ones who told us to go with the ferry as it had vastly improved with the sale. I’m not sure how long ago the sale was, nor the Trip Advisor person’s experience.