Speaking from experience, if you have to go back after many years of development, where most of the team that made the original design choices has left and want to make changes without breaking the system and discover that half of the system has no tests then it’s going to be difficult to even make the smallest of changes.

When you then decide that you need to retrofit good tests, it always becomes an exercise in trying to reverse engineer the requirements (because half the documentation has been lost or is so hopelessly out of date it may well be lost), then yes, making something well tested is very hard.

Having said that, it often worth doing it because how else are you going to have the confidence that you don’t break the system with every little change, but it very much is going to require much more effort than if it had been done in the first place.