I've noticed when I read people's advice on card balance, that they are assuming a card is being designed to be printed in a standard legal or commander legal set. That the power of the card must be such that it can't be a strictly better or strictly worse version of a card printed by wizards.
I find this type of thinking narrow minded. Card's on this site can be designed for their own isolated limited environment or constructed environment. They don't HAVE to be directly competing with other wizards' cards.
There are many cards printed, that even a strictly better version would not be competitive in legacy, or cube... However, if the card were pushed to the appropriate power level , they could be interesting cards in those formats.
Wizards have a lot of power level constraints on cards, that designers of custom cards aren't necessarily subject to.
I for example am designing a bunch of cards for my powered cube. A card equivalent in power level to swords to plowshares is a totally fine card for this environment, yet it would be utterly unprintable in standard.
Also, cards that may have a light combo synergy in a limited environment, could be totally fine, but utterly broken in constructed, where the effect can be built around. Hence, unprintable by wizards.