I would normally say "there's hundreds of pro bike racers who are at the most elite fitness and performance levels who would disagree".
But today, let me play devil's advocate......what if you live in an area where it is most convenient and safe for you to use a greenway or MUP? Safe speeds are NOT interval training speeds. Maybe it's safer and maybe fun enough to ride some kind of bike with some big knobbie tires and sucking a little more wind at a slower speed.
I disagree for hills, as with hills just push harder. No safe amount of artificial weight added to hills turns hills to mountain climbs. I did the kid trailer thing, but didn't care for the dynamics/handling and safety aspect of having a kid with me trying to train. That should be about the kid, not me. So hills, just push harder.
Time trial training must be done in position on your race bike or a TT bike with the same fit coordinates. There's only so much slower something like gators and box section wheels matter. But for cross wind you want to learn to ride your deep section wheels confidently in training. So, you just get used to accumulating a lot of miles on the same training routes over and over.
Add some gravel to go slower also. I like my long uninterrupted gravel rides. Might avg 16mph for 3 hours. On the TT bike I'd make it nearly 72mi in 3 hours on that thing on a flat route. That gets to be some big routes, or some repetition. That's 48mi versus 72mi.