Does the Men of Science event have unstated effects?

Having negative piety gives you a bonus to technology rate and taxes, which is how you get the tech rate. There is no unmentioned bonus. Ideally you want to keep piety at one end of the spectrum or the other, both have advantages. Piety is not like Piety in crusader kings, where it is all good. Instead it represents the advantages of secularism (tech, church taxes) versus theocracy (morale and stability).

You can see your current modifiers by hovering over the Current Piety label or percentage slider on the Religion tab. Hover over the flag icons on either end of the slider to see the maximum available modifiers.

As Lawton states, the options during event Men of Science have no unmentioned effects.

While there are some events that have hidden effects (= effects that are not mentioned in the description), taking a look at the corresponding event file for this particular event shows that this is not the case for Men of Science:

From \events\MuslimPietyEvents.txt:

# Men of Science
country_event = {
    id = muslim_piety.9
    title = muslim_piety.9.t
    desc = muslim_piety.9.d
    picture = INVENTION_eventPicture

    is_triggered_only = yes

    trigger = {
        religion_group = muslim
    }

    mean_time_to_happen = {
        days = 1
    }

    option = {
        name = muslim_piety.9.a
        add_piety = -0.25
    }
    option = {
        name = muslim_piety.9.b
        add_piety = 0.25
    }
}

As you can see both choosable options do exactly what they say: Add or subtract 25% of your piety.