The big problem with work for hire as a contractor is that most projects include substantial amounts of code that belongs to frameworks and libraries which the contractor has developed for whatever specialty he typically writes for. When the client insists on copyright transfer (rather than licensing) of all code that is delivered, it means that the code will have to be rewritten from scratch at great time and money cost, or that the contractor will have to lose all his preexisting IP, often developed over decades, which is plainly unacceptable unless he is retiring or cashing out and selling his whole company to someone else.
Most clients understand this. What they really need is a license to the software, which can come with things like the code, a license to modify the code, a license to sell or transfer the code license, etc, but none which deprives the contractor of his cumulative work. Clueless and overreaching lawyers though will often give advice like "only a complete sale of copyright is acceptable", which basically means you're going to be employing clueless inexperienced contractors, dishonest ones who will "sell" you open source stuff they've found on the net, or will be weighed down by the enormous time and money cost of reimplementing everything from scratch. Clauses such as "including but not limited to [work done] during the duration of the Project", we see a contract written not by a lawyer who is simply clueless about development realities, we instead see a bad faith contract that specifically intends to grab ownership of preexisting frameworks and libraries.
If your client puts this term in the contract, walk away.