Is it worthwhile to spend dust + candies in chansey?

A determined attacker will clear her out regardless. However if you're looking to stall for say, an ex raid for example, maxed out high IV Chansey can make a difference.

Blissey is the only defender with battle won occasionally. A level 38 Blissey makes a difference compared to a level 30 Blissey in that sense. However, don't expect gyms to remain much longer your colour because of the investment.

I maxed out a blissey to my current level (3137) to put in a gym to defend for an ex raid battle but could never knock out the 2600 blissey to put her in because the lower level blissey kept getting fed candies. It may have been an unwise investment but I do believe it helps to get a gym started because by the time the cp starts to dwindle there are usually 4-5 other mons in the gym.

Having at least one high level Blissey can be worthwhile to try to hold a gym before a raid, and maybe a Chansey if you want to be able to double down on a gym that already has a Blissey. Just make sure there is a flying, psychic, or poison pokemon in between so Machamp has to switch.

I also find that starting a gym with a big Blissey is as good a sign as any to other players to add their big stuff and not small throw-away gym fodder.