Yeah I would echo exactly what @Peter_G and @Revels have said above.
Surely the starting point should be maximum utility, with limitations only introduced if necessary.
My primary reason for opening my Mondo account and staying with Monzo ever since has been to keep track of my spending. Custom categories are extremely useful for that but too limited for me at 50.
@tjvr has said (A big update to connected accounts - #43 by tjvr) there is no particular reason for the current limit… so if any customers at all are asking for a greater limit, and I assume you must have metrics showing more than 1 customer (me) having hit the 50 category limit, why would you not increase it?