Soon, all metallic piping will have restrictions on sale.
You could make a bomb with plastic piping, but the shrapnel wouldn’t be nearly as dangerous.
The IRA would supplement theirs with roofing nails and ball bearings.
No, the problem with plastic pipe is that it will split before the explosive builds up much pressure. The point of an iron or steel pipe is to "hold the explosion together" until it builds up to a high level of pressure.
This of course is true only for gunpowder bombs that deflagrate instead of detonate. Explosives that detonate, like C-4, don't need to be confined. However, they should be tamped, as with a sandbag, if possible, as this directs the blast in the desired direction.