Most professional pollers do a good job.

When they are 3 or 4 percentage points off they are ripped to shreds.

The polls for the recent Brazil elections were about 49% for Lula, the result was 52%, that's a 3 point difference, not a massive error margin. With brexit the polling pointed to about 52% for remain, actual result was 48%.

But those are proper polling companies, not worthless online ones.