On original topic - one reason why Mushi may be so popular is because the bullet patterns are far more straightforward than many other modern shmups, especially with respect to folding bullet patterns. (There are a few more in ultra mode, though.)
A player doesn't need to already be aware of what's coming to be able to discern what's safe in a barrage. Despite the concept seemingly being similar to past Cave shmups, the danger comes quite differently, even the instances of bullet crossover are much more clear. (And as with other Cave shmups, Mushi doesn't t have the superfast must-memorize Psikyo barrages.)
Of course, that's not to say it isn't difficult - but it's less difficult in the perception realm and more difficult in the precision realm, methinks. Of course, the above comments apply to different degrees depending on the mode you choose.