I see 2 purposes for Bella Ld.
1. Completing the collection.
2. Possibly using it to unlock a future legendary card.
In my opinion the first wave of legendary cards will be rather weak (with few exceptions). But I hope the second wave of legends will be really decent, worth doing all the missions.
I have been on the losing side of Bella Ld, she is not terrible, very conditional, but not terrible. You get hit with Edd, for only 4 Pillz, then they lose with Spiaghi and you have her looming she can kil you with fury, I lost to a nice combo deck with her, Spiaghi, Dhagg, and the -2 life Vortex whose name escapes me. Basically hit me with Dhagg, used the lifer guy second, lost with Spiaghi and had her ability for 5 damage plus the 7 I took from the Vortex guy, that is a very good hand.
Why so many people have a hard time doing Knock-out missions? KO with Rosa do not mean you have to give the finishing blow with her. You just have to KO with Rosa winning a round, ANY ROUND. It is double-proofed working for all those types of missions.
So if people give up on the match too bad! But as soon as the card is in your hand KO the enemy and win the round with the according card. Simpler no? And It seems that so few people realized that it can mess up with your opp strategy as he awaits Rosa in fourth round.