Writs can be profitable, but they're a little time consuming. They're the best source for gold mats to upgrade armor, and considering that guild store prices make upgrading to legendary cost $80-$100k PER PIECE, this is their primary benefit. Here's some tips, and what you can expect.

Rewards, All writs

Gold: every writ pays $350-$650 depending on their level. This is important because even if you have to but items in guild stores to complete them they can be profitable!

Surveys (except provision): special maps to "mega nodes" that provide a lot of mats. For trade skills these provide a TON of raw materials, which will provide a ton of gold mats through refining.

Rewards, Specific writs

Alchemy: 9-12 mats for alchemy, including water for the writ level.
Provision: 10-40 ingredients, a green/blue/purple recipe for the writs level. Rare drop, purple ingredient mats.
Enchanting: a soul gem and a glyph to decon for leveling. Rare drop, a Kuta.
Smith/cloth/wood: a box of lower level mats, equipment to decon/sell, a trait stone, repair kits. Rare drops, a gold mat, a glass motif fragment, a psijic ambrosia recipe fragment.

Note on gold mats: the RNG generator seems to fluctuate. At best, you get a gold mat for 1 in 3 writs. Other times, the rate drops to 1 in 6.

How to more easily complete writs:

Keep the passives in each craft together so they drop in the same zone.

alchemy: any of your characters can craft and share the potions required. The only material that results in a deficit is nirn root. You will have to farm it, or buy it in guild stores. The "break even" point is about $500 per nirn. If you can find cheaper than that, alchemy will net you gold. Even cheaper - abandon the 1 in 3 writs that ask for nirn and skip those days.

provision: your characters can share these too...or use food items crafted by other people. Carry a stack of the 6 recipes for your writ level (currently 12 recipes for top level writs) and just pick up and turn in every day.

enchant: it's a rotation of health, stam and mag glyphs. I make 5 of each and then have what I need to quickly,pick up and complete for 2 weeks at a time. Note - enchanter npcs sell every potency rune. If you run out, just buy them. The money from the writ is way more than the cost of buying runes.

Trade skills: there's a rotation of three requests for each skill. They're designed to be a mat sink, so you may need to farm/buy the materials. The break even for me is $5000/200. Meaning if you can buy a stack of the wood or metal or cloth you need for under $5k, the income exceeds the cost. Some levels of mats are hard to get...some are cheap. I do writs on 5-10 characters depending on my desire any given week. My approach is to learn what's asked for, then depending on their bag space to make items in advance...either 3 days, 6 days or 9 days worth. For example, on my mule with lots of space I'll set him up for nine days in advance. So on cloth I will craft 3 each of silk robes, pant, shoulder, sash, hat, shoes and rubedo hand, shoulder and head. Same for wood and metal. Then every day he picks up all six writs and walks them over.