Use the DMG's guidance on modifying monsters
The DMG, on pages 273 through 279, includes guidance on modifying monsters and creating new monsters, including guidelines on how to calculate a monster's effective CR. This involves breaking down the monster's capabilities into a "defensive" challenge rating and an "offensive" challenge rating, then averaging the two.
Young red shadow dragons
A young red dragon has a normal CR of 10. By following the DMG's guidance on calculating its defensive CR, we see that:
- It has 178hp, which looks up to CR 8 in the table
- It has an AC of 18, 2 points higher than the expected AC of 16 for a CR 8 creature, so bumps CR to 9
- It doesn't have lots of damage resistances or immunities, so we don't treat it as having more HP for these calculations
For the DCR and OCR to average to 10, I'll assume the offensive CR is 11.
The single biggest difference adding the Shadow Dragon template to the dragon makes is to add lots of resistances that make it very hard to hurt (maybe only resistances it has in certain conditions... but then why would it ever stick around to fight you if it wasn't in advantageous terrain?) By adding so many resistances we justify raising the dragon's effective HP for the purpose of calculating its CR, as per Step 9 of the guidance on page 277:
- 178hp is CR 8
- Multiply HP by x1.5, for a CR 9 creature with many resistances, to give ~267hp
- 267hp is CR 14 (and AC 18 is appropriate for a CR 14 creature)
Averaging DCR 14 with an OCR of 11 gives us CR 13. It's plausible that the dragon's OCR was also given a slight bump because of the way its breath weapon can instantly kill adventurers now (the changes to damage type wouldn't normally make any change to its CR), which does make it meaningfully more lethal than the base creature - but OCR 12 would also average to CR 13 here.
At any rate, the change to the young red dragon's CR when applying the shadow dragon template as given in the Monster Manual seems consistent with the DMG's guidance on calculating CR when modifying/creating monsters.
So what of the Black Dragon Wyrmling?
This is a little harder to calculate since the wyrmling's stats are at the very low end of the scale and the DMG isn't super clear about whether or not CR changes should be steps on the table or full points, but we'll give it a go:
A Shadow Black Dragon Wyrmling's defensive CR:
- 33hp maps to CR 1/8
- AC 17 is 4 points higher so bumps CR two points to 2 (2 1/8) or 1/2 (two steps up from 1/8)
- For a CR <4 creature with many resistances, multiply HP by x2, so 66hp
- 66hp maps to... CR 1/2, again bumped by the higher AC by two points - to 3 (2 1/2) or 2 (two steps up)
And offensively:
- ~21 damage per round (assuming one use of breath weapon hitting two targets and two bite attacks over a three round period, as per DMG's guidance) maps to CR 3
- +4 to hit is appropriate for CR 3 so no change
Averaging those gives us CR 3. (Note that the base creature could be CR 3 or CR 2 depending on how we calculated it - as the MM gives it CR 2, that suggests the second method of stepping on the table rather than adding entire points is the better one.)
Fact is, the shadow dragon template doesn't make the black dragon wyrmling effectively that much more durable - it's maybe twice as hard to kill as it was before on paper, but against a normal adventuring party that change in effective hit points would probably only give it one more round of life.
The change to the breath weapon is probably the most significant factor in this case, since at the low levels where a CR 2 or 3 creature is an appropriate challenge, 5d8 damage could easily one shot a character and then they're dead outright. Bumping the OCR by a point to account for this gives us DCR 3 and OCR 4 which averages to 4 (3 1/2) - so 3 to 4 would seem a reasonable CR for the shadow black dragon wyrmling.
Of course, the DMG is quick to point out that the guidelines it provides are just that, and you may need to adjust the CR of a creature if it proves to be more or less dangerous in actual play. Many monsters in the published material have challenge ratings which are slightly off from what you would calculate from the DMG's guidance because of their special abilities and features that change the effective challenge they provide.
It's almost balanced, but...
Let's look at the scores for a Kuo-Toa in the Monster Manual.
STR 13 (+1)
DEX 10 (+0)
CON 11 (+0)
INT 11 (+0)
WIS 10 (+0)
CHA 8 (-1)
Their highest ability score is actually Strength, which is neither of the ability score increases covered. If I had made this, I would have made the increases +1 Strength and +1 Constitution.
I also would have given this race a swim speed of 30 ft., as the race is literally a fish, and it is in the stat block. Most of these features are balanced out by Sunlight Sensitivity, a great, big, debuff. If anything, removing the Otherworldly Perception would make the race underpowered.
Surprisingly, this race would fit best in a melee class if the stat block was followed (barring Sunlight Sensitivity), but it seems that the creator attempted to shoehorn the monster into a spellcasting class. Otherwise, everything seems legit, as I cannot find a source that argues against it.
On the fact-based side of things, let us look at the traits from a user's standpoint.
Languages: Undercommon is not the most useful language, but good for campaigns that deal with drow and the like a lot.
Otherworldly Perception: As enemies on the Ethereal plane are rare, I'm going to ignore that. Seeing through invisibility, however, is powerful. The hole in this is that the Kuo-Toa cannot see the creature (and hence does not know whether it is an invisible Duergar or an invisible Archmage), and only knows the creature's location if it moves. This is akin to the first-level spell Detect Magic:
An aura forms around objects or creatures you can see, indicating that it is magical. It must be within 30 feet. You also learn its school of magic, if it has one.
- Sunlight Sensitivity The reason this race is balanced lies in this trait. Melee classes are killed by this feature, and (most) spellcasters don't like getting within 30 feet of a creature just to pinpoint its position. Disadvantage on Perception is hard to ignore:
"Yes! I rolled an 17 for Perception! What do I see? Oh, wait, disadvantage...nat2."
Amphibious: Saves you a few gold pieces on Potions of Water Breathing.
Slippery: Great for melee classes, especially Monks, who don't usually rely on Strength.
The reason that this is almost balanced is its way of not fitting into any particular class well (if it had the correct ABC). If you are a wizard, chances are you'll be too far away to use Otherworldly Perception. If you are a Fighter, disadvantage on attack rolls is TERRIBLE. For WIS based classes...it may fit, but they really shouldn't have these ability score increases that make them attractive for those classes. Wisdom is NOT the Kuo-Toa's strength; Strength is their strength.
Best Answer
The easiest way to avoid the penalty of the Drow on prime material plane campaigns, is to only play Drow in campaigns which take place indoors, underground, or mostly at night.
The second easiest way is to gain advantage. A Drow at 3rd level gains the spell "Faerie Fire" which grants advantage to anyone attacking the victim of the spell. If you are willing to multiclass, you can contemplate taking a level of Barbarian which will allow you to rage and get advantage on attacks, but it won't help you with perception, or 2 levels of Rogue to gain the cunning action ability to hide to gain advantage more easily. You can also try to work with spells that don't rely on attack rolls, but rather use saving throws to avoid the penalty all together.
The traditional techniques of wearing a hooded cloak, or only attacking from the shadows will not work in this edition as the text specifies that if either you or your target is in sunlight you have disadvantage.
There are two possible reasons for why only Drow get such a handicap.