So I tested the ironing function some and it's really useful. Especially with colorfabb woodfill filament (or others with wood fibers I guess) which gets a really nice top surface!
However: I tried just activating ironing on a model today and an effect I've seen earlier became a real problem. The model has three flat surfaces on different levels. After ironing a layer and moving on to print upwards the next maybe 20 seconds the filament goes really dark and weak - I'm guessing the nozzle is way too hot but I haven't checked during print to verify this. It could also be that since so little is extruded during ironing the filament in the nozzle has been liquid for a long time and started to deteriorate?
So I got the idea (first looked if it already existed but couldn't find it): A pause after ironing, to a specific temperature or for a certain time (like maybe 30 seconds). This should fix this issue. This print easily cracked in half which was a bit of a shame, it looked so good!
EDIT: Printing on a Ultimaker 2 Go, neglected to mention this.