I don't see why

Instead of everyone having to use the site during a three-hour period, they get 90 minutes at a time of their choice anywhere within a 24 hour period. That's going to spread the load, so while the site probably won't be as fast as usual, it shouldn't come to a near standstill as it did first time. Given the fact that it's going to be used by people all over the world, I can't see that there will be a time when you could be certain that not many people will be using it, but I would certainly expect it to be running at a reasonable speed.
People complain that the big family history sites are just a bunch of money grabbers, but then when they do a free offer like this, they just seem to get swamped with complaints and criticism. I think this is a great offer by FindMyPast, but I honestly couldn't blame them if they didn't offer anything similar again in the future.