You'll ultimately have to go with your heart on a decision that is right for you. But since you ask what I'd do, I'll tell you!
You have an opportunity at a cheap vacation in your hand. The Varadero opportunity is a bird in the bush. You can ultimately do both, but time is limited on one. Were it me, I'd take the free stay for $800 airfare, and plan a future visit to Varadero. The saving that you are getting, can be applied against future travel and you may be able to consider the next vacation sooner than you would have otherwise or plan to go for longer because of the good deal that you got now.
Reviews change with the wind and seasons. If the reviews have been poor lately, you can anticipate that they will change again soon. Look at the reviews over a longer period of time and see what was happening last February. Maybe some of the recent problems were because it was the low season or the countries of origin of the travelers. We have different expectations. I think we have also all had a moment where we shook our heads over the reviews offered by some and said "did they stay at the same place I did?" Go with moderate expectation, an open mind and a positive outlook and your $800 vacation should be grand. I did not read the reviews but scanned them and noted that people had either a horrible time or a great time. That says something. If you take precautions and come prepared, you won't be surprised. The bones are obviously there for a good vacation and it looks to me worth trying for $800. But I've not had a really bad vacation yet to know how bad it could be!