No downside to enroll as long as you pay on time each month.
Sign and Travel might be a bureaucratic stranglehold and potential liability to be used against you - my story
when b of a slashed credit in 08 (there are hundreds of forums dedicated to bashing them - there was no warning and they took down people who had lines for 30+ years)
My amex sign and travel was immediately rescinded.
They said I can get it back by invite only - which didn't happen and I finally cancelled all my cards in 2010 to start from scratch - which worked.
But meanwhile, whenever I tried to get an increase in my other amex cards (back to the original amount before b of a screwed everyone) they always said they can't help my account because "once sign and travel gets taken away, I can't get any new credit, etc".