Gift/Key Preference

+1

Bet there is a chance that winner may not activate key on his account and pretend the key is used

The other way around. Humble tags people who are using keys instead of using their gifting system. Everyone who complained about them eventually coughed up that they used keys instead of links since they were too lazy to click two more times.

^ This.

There are drawbacks to everything, however, as Juzer pointed out, above.

get support if needed

Until hb flags you as reseller :)

I forgot to link.

SteamDigitalGiftCard
https://store.steampowered.com/digitalgiftcards/selectgiftcard
Humble Bundle Support
https://support.humblebundle.com/hc/en-us
Steam Database · SteamDB
https://steamdb.info/

Gifted spare keys from a bundle do not include DRMfree variants or soundtracks, those stay on the original purchaser's account. (Unless thats changed in the last year or so.)

As Ratha already touched on:
DRM-free content is considered part of the overall bundle package. So if you gift an entire bundle, they'll properly be provided to the gift recipient. However, if you're gifting an individual key, the extras have already been redeemed to the purchasing account (meaning they can no longer be transferred).

Flip side, that's a good way of essentially duplicating a game [in circumstances where both a drm-free and steam copy of the base game are provided]. Which, putting aside the circumstances where there are potential ethical issues to the matter, can actually resolve certain concerns some purchasers may have. (Though, conversely, it emphasizes how weird it is that Humble doesn't associate drm-free content activations with the associated Steam key reveal.)

For example, if you're going by EU laws [which now fully stands by the right of first sale for all goods, something which US lobbying has restricted from digital goods*] and you only intend to play the game once yourself, playing the drm-free yourself and trading/gifting off the steam key is a pretty decent workaround for the current Steam circumstances.

*This initially caused by way of lobbyists manipulating digital security laws to also cover drm-protected content, and then later, from that, to have digital goods restricted in general; Ironically, as the EU has realized, drm-protection actually ensures that the content can properly be transferred, meaning it'd be a legal violation to not allow for such. Meanwhile, drm-free content should actually be restricted, given that it so easily can open itself up to the unauthorized reproduction restriction within copyright laws.

Downside, the drm copies Humble offers aren't typically updated further (though some are), meaning that the games may eventually become outdated or lose you access to free enhanced edition updates on Steam. As far as soundtracks/etc go, if you really wanted to, you could just download those and then send them to the recipient on your own. That's about the only real way to approach that, though.