I think more thought often needs to be given to the type of seeds that are put into seed bombs , if mostly annuals are used ie field poppies , cornflower etc the first year display may well be colourful and give a great sense of achievement but future years may yield nothing at all . This is due to seeds like poppy needing to have the soil disturbed for them to continue to germinate in subsequent years.
personally i always add `meadow cranesbill`and `california poppys`
the cranesbill is a really tough plant that will grow most places and has the added advantage of projecting its seeds over a distance like the california poppy by means of pods that spring open.cranesbill will take 2-3 years before coming into full bloom but i planted some 25 years ago in a wasteground area and they are still going strong , providing me with loads of free seeds every year , good to get going in yogurt pots too. geranium pratense and eschscholzia.