What I find funny about Cosima is how it seems like the majority of people build around the absolutely god-awful back half of the card, instead of the pretty-dang-strong front half.
The issue with the back half...well, one of them...is that in mono-blue it's pretty hard to enable playing multiple lands in a turn, which means your absolute maximum value is basically drawing 1 land per turn, which isn't even worth a full card since it renders drawing your own lands significantly less valuable. And since the Omenkeel is just a vanilla 3/3, you need to actually put some effort into enabling this extremely mediocre payoff. And then to add insult to injury, stealing lands kinda sucks because when people leave the game they'll take their lands with them. As far as I can tell, people just like the novelty of a mono-blue vehicle payoff even if the payoff is trash.
Cosima herself is a pretty solid if confusing commander. My version basically played as a heavy blue control deck - lots of (mostly artifact-based) board wipes, counterspells, etc, alongside nearly every available fetchland. For the killing blow, you pop her back into play with 19 counters (making her 21 power), ideally EOT with a fetchland having just wiped the board, drawing an insane number of cards to protect your wincon and then smashing people for an easy 21-damage commander hit while controlling the board with a brutal iron fist.
What makes her really strong, imo, is that she's almost impossible to stop as long as she's not removed immediately - and who the hell would remove her that quickly? She's mono-color and pretty obscure - not even in the top 1000 on EDHrec - and most people build her as a bottomlessly-terrible vehicle tribal deck (well, more than half at least -
Smuggler's Copter at 57% currently). So as long as she's not killed ASAP she's exiled and basically untouchable until you decide to put her back into play, an action which automatically puts a billion cards in your hand to deflect any potential incoming answers. And even if someone has a stifle, it's basically worthless - countering her trigger just stops her from entering on that particular landfall, it doesn't strand her in exile or reset her counters or stop all the card draw. She can just try to enter again the next time a land enters the battlefield. So they'd need to be able to stifle every turn, if not multiple times per turn with fetchlands.
The biggest weakness is that she doesn't contribute anything until she does re-enter, but in a pinch you could always bring her back early for some mid-game draw and then exile her again. That should re-stock you on fetches at least.
Anyway, underrated commander for sure, imo. Seeing her kinda makes me want to rebuild her.