I live in an area with pretty much continuous short rolling hills like what you’re describing and did a fair amount of outdoor workouts last year (and the last 2 weeks as we’re getting spring weather in the Carolinas. Couple thoughts that I have:
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There’s absolutely value in doing steady/sustained workouts outside even if your power fluctuates, especially if your target type of riding involves sustained power. It’s an important skill to be able to control your power on variable terrain or in changes of pace, especially if your doing any longer events or rides. To you exact point, 2 20’ intervals may have the same NP, but one where you’re constantly spiking into VO2 range is going to be much more costly than keeping the power in say a 20% range.
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One of the best ways to control you power spikes is to anticipate the terrain, and adjust your power/shift proactively. This means downshifting right before you hit an incline and shifting down early enough that you never feel excess strain on your legs going up a roller- conversely, shift up a gear and focus on pushing down right as you are cresting a hill to avoid having your power drop on the downside. My NP is always a little lower on rolling terrain than flat because I avoid having spikes which take me way above my target zone. But you make up for that because outdoor workouts are usually longer including time to ride to and frim your spot for intervals.
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There’s some intervals that you’re just better off doing on the trainer. For example, over/unders can be tough to do on rolling terrain, especially if you have steep uphills and downhills. Any workout where you’re trying to stay in a very tight power range is also probably better on the trainer (think intervals just over threshold like Stromlo or Black Hawk)… Depending on your event (like a TT), you may want to do them outside as it gets closer to practice dialing in your power, but early in the season, you’ll spend more time at your goal intensity indoors.
As an example, here’s a workout I did outside on consistent rolling terrain on my TT bike- targeted one hour at 90-94% FTP. You see some variation (the drops were from the couple busy roads I had to cross), but I’m largely right within that range.
Here’s McAdie +1 outside last week. Again some variation, but got pretty close on all the intervals with proactive shifting. I did choose a flatter route for the first 2 intervals which is why they look better.
Hope this helps!