My problem is with the D&D 5e paradigm 'spells do only as they say they do'. If I am reading flock of familiars1 correctly, then already possessing a familiar weakens this spell, to the point where you'd have been better off not having that original familiar. The familiars summoned by Flock of Familiars appear to be much stronger than those summoned by Find Familiar because the spell Flock of Familiars does not state that one would be deaf and blind to their own senses.
Since possessing a regular familiar would cause Flock of Familiars to summon one familiar less, it effectively turns three strong familiars into two strong familiars and one weak familiar.
The spell description of the Flock of Familiars spell states (emphasis mine):
If you already have a familiar conjured by the Find Familiar spell or similar means, then one fewer familiars are conjured by this spell.
Familiars summoned by this spell can telepathically communicate with you and share their visual or auditory senses while they are within 1 mile of you.
Whereas the spell description of the spell Find Familiar states (emphasis mine):
While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar’s eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses.
RAW, does the Flock of Familiars spell create familiars that allow you to see both through their and your own senses at once?
1 From the digitally published adventure, Lost Laboratory of Kwalish
You say 'spells do only as they say they do', but disregard where the spell says this:
If you do not read this as, "all the rules in Find Familiar apply except where they are explicitly overridden here", then your Flock can do very little - you can communicate with them, and cast spells through them, but nothing else - eg, nothing says that they can take actions in combat (or how).
If you accept that they can do all the normal Familiar stuff, just that there are more of them and they have a longer range (as explicitly specified), then you accept that they also have the same limitations - eg, blindness & deafness while perceiving through their senses.