When I look up (as an example, but is true for all mana values) 'all black, modern format, cards with converted mana cost of 1'
The database doesn't understand anything with multi coloured, split colored, or even some of the "x" values.
I.e. it is showing me cards with all sorts of converted mana cost, Beseech the Queen for example has a cost of 3 which can be played with either 3 x black mana or 3x two colourless mana, it certainly does not have a converted mana cost of 1!!, nor does Black Sun's Zenith have a CMC of 1...Etc.
Any card with either a split mana, like Slitherhead, or with other uncommon mana costs and 'x' are all coming up with the wrong converted mana cost.

Secondly. Multicoloured cards, there is literally no way to tell the database to show everything that can be played with just black mana, it Always either includes every multicoloured card including those with non-optional other colours, and if tell it to exclude any card with another colour it doesn't just exclude the ones with mandatory other colours, but anything optional, so for example if I tell it to exclude green, so I don't see cards like Lotleth Troll, because that requires mandatory green mana, then it will always exclude Slitherhead because of it's optional green mana... Hence making the feature entirely redundant.
If I want to see what I can cast for my two black mana I have to either wade through countless multicoloured cards I can't cast just to pick out the ones I can, or if I try exclude those, then it will also exclude ones I can cast because it can't differentiate between mandatory and optional costs, this is aside the numerous cards which cost 4 , 5 or whatever mana which literally shouldn't be in the 2 cmc list

Lastly if I reorder the output from a search to say put a set of creatures in order of power or toughness, many of them will disappear from the list, for example when I looked up one mana black creatures and reordered them in power, will'o'the'wisp simply disappeared off the list, rather than being at the end with zero power.

I don't know where else to report these issues, your app simply sends me here.