For a young working adult in his late 20s, is 100% REITs portfolio viable option to dividend investing?

Do not put your eggs in one basket. One day if the market is unfavourable towards reits, your portfolio will suffer a huge paper loss. You must ask yourself if you are able to tolerate such losses.

If you're intending for a full 100% REITS portfolio, combine it different sectors of REITS. E.g. 1.Healthcare REITS, 2.Hospitality REITS, 3.Retail REITS. Healthcare REITS are considered the most stable. Hospitality REITS are highly sensitive to economy and very cyclical. Retail REITS is easiest to appreciate whether doing well anot. Read more about REITS. And then combine different sectors of REITS to make a 100% portfolio for constant dividends.