I've said for some years now that I have no hate for authors who take on work-for-hire contracts to write tie-in novels for popular properties. Some folks I first knew as tabletop RPG people used that opportunity to build up the skills and personal brand needed to successfully transition to selling their original works (because we can't all be Scott "My editor discovered me by reading my Livejournal." Lynch), such as Jeff Grubb- and similarly, I first read Mike Stackpole and R.A. Salvatore through their gaming tie-ins (BattleTech and Forgotten Realms, respectively).
Timothy Zahn? First encountered him when he wrote Heir to the Empire, and gave us the glory that is Grand Admiral Thrawn about 25 years ago. (Never been a big reader of the tie-ins for Star Wars, but I'm familiar with the now-disavowed corpus that was the Expanded Universe due its ties to--and outgrowth from--the original West End Games version of the RPG.) So making tie-ins didn't hurt him, and there is one big reason why it worked: Zahn's tie-ins were faithful, fun, and satisfying reads that franchise fans wanted.
So yeah, you can't just half-ass it and expect things to be golden. You have bosses--and I don't mean the readers--and they have a brand to protect (if they are at all competent), so there's homework to do and meta-narrative concerns to satisfy in addition to just writing a good story, but so long as you can be a team player as well as a good storyteller these tie-ins can help cover two ongoing concerns: making a living, and building your personal brand.
Yes, you should take all the precautions that you would for any other contract offer, and you had damn well be ready and able to walk if that offer doesn't measure up, but if you get a decent deal and your alternative action plan isn't going to give you better results during that same time and resources spent, why not? What you learn therein you can--and should--apply to putting out your own original works.
(Lucasfilm, start using your position to find new talent and break them out writing tie-ins. Your go-tos are not helping with the SJW bullshit they're slipping in, and that also means you have some serious rot in your offices because they aren't doing their job of keep that out.)