Sunlight Sensitivity might require mitigation, but not for casters.
You have disadvantage on attack rolls and on Wisdom (Perception) checks that rely on sight when you, the target of your attack, or whatever you are trying to perceive is in direct sunlight.
Compared to other Elves, Drow have more racial traits and, in particular, Superior Darkvision and Drow Magic are quite powerful, so Sunlight Sensitivity is meant to balance this difference.
In practice Sunlight Sensitivity only provides significant drawbacks to martial Drows, not caster Drows. To see why, lets break down the trait's two effects and the condition under which they apply.
Disadvantage on Perception Checks
Disadvantage is generally acknowledged to be equivalent to a -5 bonus, but the Drow PC has proficiency in the check anyway so even under direct sunlight they will not be completely inept. Moreover, in my experience it is not a significant problem because one of these two things tends to happen:
- the Drow PC chooses to help an ally's perception check instead of rolling their own.
- a party member succeeds in the same Perception check, thereby nullifying the Drow PC's failure.
Disadvantage on Attacks
Consider that (at the time of writing) only 11 cantrips and 23 spells of 1st level or higher require the caster to make an attack roll. Drow casters can simply choose not to learn these spells and they still have plenty of other good spells to choose from (including offensive ones), so much so that Drow casters are just as effective as any other caster, even while fighting in direct sunlight.
Martial characters are those that rely primarily on weapon attacks to be effective combatants, so by definition making attacks at disadvantage is a significant drawback for these character. As such, Martial Drow are significantly weaker than other martial characters when fighting in direct sunlight. However, it's worth noting that both faerie fire and darkness can undo the disadvantage, so Drow Magic provides the tools to handle one or two combats in direct sunlight.
When Sunlight Sensitivity applies
How often sunlight sensitivity comes up varies significantly from campaign to campaign, but in my experience the most significant parts of the campaign seldom take place in direct sunlight:
- dungeons, caves, and typical buildings are obviously not a problem.
- streets, forests, and jungles ought to have plenty of shade at any time but midday.
- plazas, thoroughfares, and sparsely-wooded wilderness are the kind of places on which sun shines the most.
Finally, don't forget that as a DM you have full control over the weather and the direction the sun is coming from, as well as some control over the time of day. This control is all the mitigation you need, for example if the Drow PC is struggling during a cross-country trek you can just banish the sunlight with some bad weather, or push most encounters to occur at nighttime.
Manage player expectation in session 0
At the end of the day, Sunlight Sensitivity is only a big problem for martial Drow (and only if the DM chooses to make it into a big problem), whereas it's not much of a problem for caster Drow. Nonetheless, if a player is thinking of playing a Drow you should talk to them about how often you think Sunlight Sensitivity will come into play, so they can make an informed decision.