Just because driders are more powerful doesn't mean they aren't cursed. If you go with my suggestion above, that power comes at the price of being permanently enslaved to one of Lolth's bebilith minions; it's the bebilith that gets to wield the power, not you. I can think of other possibilities. A drider's existence could be full of agonizing pain. It might have its lifespan cut to the lifespan of a spider (20-30 years for the longest-lived tarantulas, 1-2 years for most species). It might have its soul ripped out of its body and a demon put in its place. It might be afflicted with madness. There are all sorts of options.
Certainly there are all sorts of options, and I'm in favor of having options. I'll be honest that I don't particularly like your option, at least for players. I tend to frown on anything that affects "attitude adjustments" on players because either A: you essentially take over that character (which admitted is an option, and IMO, a better one) or B: you come up with a new "lawful stupid" situation, where you force a player to play in a manner they don't enjoy. Neither are particularly enjoyable concepts for the player or the DM (at least not
IMO).
I still like the duality of it being a blessing and a curse, because that just strikes me a Lloth way of doing things. I think there should be a standard "here's the bennies of being a Drider" template, and then some additional
optional penalties, such as madness (lost control in some manner and become an NPC), thirst for the flesh of the living, agonizing pain, shortened lifespan; provide some examples but otherwise leave these sorts of things up to the DM to determine the specifics of.
Gaining the Drider template really shouldn't be a common thing for Drow players, so I really think it's not something that needs to be explicit. The player and the DM should work out the details of what's gonna happen, what the player needs to go through, and what success or failure means.
I think Wizards did a fairly good job with the "Drider Template" here:
https://www.wizards.com/default.asp?x=dnd/mm/20011118a
If the DM wants to make it to be a blessing, he can give it as is. If the DM wants to make it punishment, they can add some penalties to it. It's not something I expect to appear in Core, so hopefully Driders will be uncommon to rare beings that serve a special role in the game. At what like, 8th level when a Drow can take the Test of Lloth, a +2 CR isn't that big of a deal. Your characters are already superior to a vast number of people and creatures in any land. Taking on a position as favored of Lloth or dreaded monster is a fine thing to do around 8th level.