It's actually quite logical that the slingshot effect would be difficult to pull off. Not only is it dramatically and logically necessary if we don't want the timeline to degenerate into chaos (because if slingshotting were easy, any idiot with a warp shuttle could wipe out all recorded history on a drunken dare), but it's consistent with what actual physics says about the principle. The mechanism that Star Trek calls the slingshot effect was described by physicist Frank Tipler in a seminal 1974 paper (notably, ST got there first by seven years), and the physics behind it has been the subject of many research papers in the decades since. According to General Relativity, such a time warp (indeed, pretty much any time warp) would have a divergent stress-energy tensor -- effectively a runaway feedback effect that would cause it to emit ever-increasing radiation and vaporize anything that tried to pass through, or just collapse the warp itself the instant anything tried to pass through. (So when I mentioned this in the DTI novels, I wasn't making it up. It's the real deal.)
So the needs of storytelling and simple logic dictate that slingshots shouldn't be easy, and the laws of physics conveniently agree that they should be next to impossible. So I had plenty of very good reason to go with that assumption. TOS portrayed it as easy because that served the particular stories they were telling those weeks, but it better serves the continuity as a whole to assume it isn't easy. After all, we have never seen a 24th-century starship crew attempt a slingshot maneuver. There has to be a reason for that, and physics provides one. It makes sense to assume that the ability of Kirk's crew to compute a successful slingshot effect was rare. Yes, it's implausible that nobody else was able to replicate it, but the alternative is far more implausible for multiple reasons.
http://en.memory-alpha.org/wiki/Time_travel_episodes
TOS - 5
TAS - 1 (although I'd include The Counter Clock incident and Jihad)
TNG - 12
DS9 - 11
VOY - 12
ENT - 9
Movies - 4
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Total - 58 (or 60 with the two TAS added)
And that's just for 5 ships/crews. What about the rest of Starfleet not to mention the Klingons, Romulans, Tholians, etc? Add the novels and people are popping through time fairly regularly.
You know, you'd be a lot less handicapped in this conversation if you'd actually read my novels. For that matter, you'd be a lot less handicapped if you'd actually pay attention to the earlier posts I've already made in this thread. What you're ignoring, and what I've already pointed out, is that most of the time travels listed there were due to cosmic phenomena, alien intervention, future time travellers' intervention, and accidents rather than the controlled, replicable efforts of the Starfleet characters themselves. I've already spelled out the reasons why it's only sensible to treat time travel as rare and difficult, so most time-travel episodes are predicated on making the time travel result from circumstances that are not easily replicated. Stories that fail to do so, that make it seem easy, are thereby making a mistake or an oversight. It's nothing to be embraced.
Check out Timemaster by Robert L. Forward sometime. It's even got a technical appendix.
I read that book over a decade ago, thank you very much. And if you were acquainted with Watching the Clock's appendix, you'd know how much other research I did, both in hard SF and real physics, in putting together the book. Many of my sources are also listed in my annotations for WTC.
Except that the slingshot effect is relatively easy to pull off. It happened to the Enterprise once by accident and then they used it 5 times, once to get back the first time and twice for two way travel. Spock computed it so well in his mind that not only did they end up right when they needed but they crashed right outside Starfleet's front door. No mention was made of excessive radiation. Attempting to say that Trek actually worked out a divergent stress-energy tensor before a physicist did is just retconning. They just needed a way for the ship and crew to get into the past. It's about as realistic as the method the Planet Express ship used in Roswell that Ends Well. It comes down to "We need to get to the past. Start your calculations for time travel. <poof>. We're there." Welding the fictional requirements of treknology onto theoretical physics is trying to solve a problem that doesn't exist. If they need to travel in time for the story then they'll travel in time.
My point of listing the time travel episodes was simply to show that it's not a rare occurrence regardless of how it's accomplished. If the Enterprise-D/E crew travelled through time a dozen times (more counting the novels) is it unlikely that other ships haven't as well? Perhaps more, perhaps less but it's likely that a large percentage of Starfleet have travelled at least once in their careers. There's more going on in the galaxy than just the episodes we see. Claiming that time travel MUST be difficult and we're only seeing the times that the crew aren't killed off by radiation makes no sense. If Picard can do it so can another captain. Trek has shown us multiple times that time travel is not unusual and at times it's even relatively simple to pull off.
I read the first DTI novel. I haven't read the second one yet but I don't feel handicapped in the least either way. Your "rules" on time travel will last exactly as long as we don't get another time travel episode or movie or novel (assuming the writer of the novel doesn't feel constrained by your time travel rules.) It seems to me that you're putting a lot of effort into explaining something that doesn't need to be explained. Strictly my opinion of course. Many other people here disagree. After all, it's ALL make believe.