Loyal units are aplenty and don't have any upkeep costs.
The main campaign absolutely encourages you to abuse your loyal units and promote them to tier3 or tier4. Losing a loyal unit is devastating however, because you pretty much only get one of each type ever (except the mermen: IIRC you get like 6 loyal mermen in the default campaign).
I remember playing Northern Rebirth just yet, I think I had a few loyal dwarves but that's all. Of course you also have a core band of units which come up in every scenario. They are often irreplenishable, such as there's being only one wizard in the whole campaign.
You need to be extra careful to keep loyal/special units from harm, and for the most important ones it's an explicit scenario requirement. And that means regular career units doing the heavy lifting.
Experience is only offered upon killing units, or in battles. Either way, it means moving a Loyal unit to the frontlines to "feed" them experience.
The risk is inevitable. If the loyal unit misses, and you have a bad turn (Ex: Elf Shaman missed the slow attempt), then you just hope the AI also has a bad turn on the counter-attack. If anything, you need to do this earlier rather than later (so that your Loyal units reach Tier3 or Tier4 at a faster rate, so you really take advantage of the Loyalty bonus).
Your slow will eventually miss. Wraiths and other undead will "luckily" self-heal themselves to max HP. Orc Warriors (A common Tier2 Orc) will "luckily" one-shot your (lower-HP) Tier2 units, and royally mess up your line. Your opponent will get breakthroughs as a result, and your loyal units will die. Even if you optimized your unit placement on the highest evasion tiles (70%), with the highest HP values with the maximum support across the line (ie: positioned in such a way that only 2 units can ever attack one unit), these events will happen.
Whether you choose to restart-from-last-save at this point is up to you, but the next chapter will only be more difficult.
Luck is part of Wesnoth. Period. I can complain about it, but the devs have made it clear that this is exactly how they want the game to be.
The main campaign absolutely encourages you to abuse your loyal units and promote them to tier3 or tier4. Losing a loyal unit is devastating however, because you pretty much only get one of each type ever (except the mermen: IIRC you get like 6 loyal mermen in the default campaign).