On my conworld, I am developing a creature that is covered externally in taste buds. What would be the evolutionary advantages and disadvantages of having this adaptation?
Taste buds and olfaction are essentially the same: detection of environmental chemicals at concentrations unlikely to be toxic.
Advantage 1: Directionality. It would be easy to determine a concentration gradient across your body and so identify the source of a chemical.
Advantage 2: Opportunity. This is a large sensory apparatus and so this redundancy improves the odds of detecting a very weak signal or one present only at one area of the body.
Advantage 3: Redundancy. If you are in a situation where you are likely to lose a body part or have it taken out of commission somehow, you will not lose the ability to smell / taste.
Disadvantage: Cost. This is usually the answer to "why not have more of X?". Sensory apparatus and the neural capacity to support it are energetically costly. I found this analysis for the energetic cost of vision for a fish (which has eyeless and eyed varieties).
We show that the eyes and optic tectum represent significant metabolic costs in the eyed phenotype. The cost of vision was calculated to be 15% of resting metabolism for a 1-g fish, decreasing to 5% in an 8.5-g fish as relative eye and brain size declined during growth
You would want full body tastebuds in some environmental situation where one of the advantages gave you a caloric payoff at least enough to compensate for the energetic cost. Or a fitness payoff - energetically costly is fine if it gives you an edge at finding a mate.
Chemosensing? Every advantage. Early warning predator system, find prey easily, signal and receive status of fellow organisms. It'd be like a dog's nose on steroids.
The real question is "Where would this likely evolve?"
Tracking - Just by touching the ground, you could follow someone.
Walking into a public restroom