A lot of things factor in.
Plumbing obstruction by accumulation is often a slow-moving process, and sometimes isn't an issue if people take the right precautions or have the right habits.
The big thing is to never let lipids of any type down the drain, grease, lard, oil, all of it is your biggest danger to congeal and start the process.
Next is to keep regular liquid traffic. If you're sending a bath tub full of hot water, or a sinkful of near-scalding water daily, you're going to clean a lot out on a maintenance basis.
Next is to make sure things that go through the disposal will biodegrade easily. If it breaks down quickly it won't get caught up permanently in the growing grease clog.
Then people are relying on draino, and hoping that the day doesn't come they have to roto-rooter the whole system out.
Thing is, once those grease 'balls' start, it's a near inevitability. Grease isn't something that just washes away. There was a news thing on how in the London sewers, they're pulling out balls the size of a motorcycle that's all hair, grease and wet wipes.
EDIT - revise that, they've pulled out one single ball weighing in at 10 TONS.
https://www.theguardian.com/uk-news/...a-sewer-london