How difficult is it to fit a lower denture to gums with bad undercuts ?

Depends: Depends on the severity of the undercuts. The undercuts can be managed by surgical reducing the severity or by using bone grafts to fill in the undercuts. Without doing one of these 2 options it would be difficult to make the denture stable and the denture will be very loose to avoid engaging the undercuts.

Difficult indeed: May need a long term soft liner which some companies make for us.

Consider implants: Regardless of your gum shape and size, while soft reline materials might be one interim solution, the only real way to get true retention and close to natural eating ability is to have an implant supported denture. Every other technique belongs in the 19th century!

Very: Bad undercuts are areas that will rub and create alot of soreness and pain. Dentures require 'draw' meaning you have to be able to easily place and remove them. Undercuts compromise both and should be removed.

It Depends: Undercuts can actually be useful in retaining the denture. Having your dentist adjust the plastic to make you comfortable is important.

Depends: That depends on the severity of the undercuts. Typically, severe undercuts in the jaw would have to be surgically eliminated or decreased to prevent irritation to the gums upon placement or removal. Your dentist will determine if that is needed.