Somehow I think this shard benefits mono decks much more than rainbow... and I guess that was intended by Zanz. Ok, using it on a flying eternity, butterfly creature, some nymph or anubis can help a rainbow, but won't affect the winning percentage much, and the playing style won't also change dramatically.
Right now a weakness of the rainbow FFQ/Fallen Druid is that they compete for quanta. Zap--no problem anymore. I also feel this would most benefit the Butterfly Effect since it's a costly ability in a based deck.
Not sure how I feel about the card at the moment. It'd open up some new options, but remove the need for splash decks altogether.
Splash decks would still be more consistent (not reliant on drawing a SoR) and as long as you don't face devourers/black holes there is little advantage in using a SoR (you don't have to pay for the ability, but you pay 3 for the shard, you can use the ability 1 turn earlier, but you have to replace one card in your deck with a SoR), overall not much of an advantage over a splash deck. Maybe it would be a little faster in the beginning due to the right mark, but nothing more. Also splashing would still be necessary to use spells/permanents.
The biggest difference is that you can make mono decks that would need 3 or more elements without a SoR. For example anubis+scarabs needs
but with SoR you can make it with
only. You could even splash
to use PA on your precious permanents and plate armor on your scarabs, so you basically run a four element deck but use only 2 types of quanta.
Also SoR is useful with expensive abilities like nymphs' and arctic squids, flying weapons... but you usually do not splash for those
As for SoR and rainbows, for quite long I've been playing with pharaoh instead of queen, but from what I remember the
conflict between queen and fallen druid was rarely game changing. Also having to wait for a SoR changes a queen+quint combo into queen+SoR+quint combo which could be a problem.