I haven't read the bts books on those shows, but just by virtue of the FX being an in-house proposition, it had to have been easier in a lot of ways. They had longterm studio folks who had been doing this kind of work and other kinds of miracles for decades, so while the volume might have been heavy, they didn't have to reinvent the wheel. And it shows, because a lot of the shots in TIME TUNNEL, plus the saucer going down prior to crashing in LIS and the flying sub doing its thing were pretty hot FX moments.
With the exception of one book on The Time Tunnel, all of the books on the Irwin Allen shows are fan produced with no real look into the UCLA files. So we learn practically nothing other than pointless trivia and fan commentary.
However, THE TIME TUNNEL - A History of the Television Series
did go into the archives and give a pretty good idea of what went on and it wasn't nearly the uphill climb Star Trek endured. Stock footage was no problem, all of the futuristic sets were redresses and odds and ends from the Fox lot, with the main issue being the scripts. The story editors would try to get the outlines into episodes to be filmed under budget but comprehensible. There were one or two very insightful editors who would point out every plot hole. Depending on the episode, the cost and/or Irwin's mood, the holes would either be fixed or ignored.