I know there is a no sympathizer variant. It seems that using the sympathizer pretty much allows the humans to determine who wins the game right there (since it is such a huge shift in number of players from one team to another). Is that pretty much the opinion here?

No.

For the sympathiser to be human one of the resources needs to be red which then gives the cylon a good start for the 2nd half.

Also, if a cylon was present from the start then he could try and stop this strategy or ensure other resources were low to so the humans couldn't use them to protect the 'red' resource.

Probably the only case when the humans would not be at risk doing this would be when the cylon comes out in the 2nd half so there would have been no sabotage in the 1st half.

But then players have to think that they may be a cylon in a game, and don't want to give the humans to big an advantage that has to be clawed back in the 2nd half when they go metallic.