I’ve done SSBII a couple times now and each time it has been a major shock to the system. The sweetspot intervals are often more like threshold intervals because you’re spending your time at the top of the zone rather than the bottom like in SSBI. And the jolt to your VO2 system can make those interval days insanely hard to get on the bike mentally. I found that SSBII is a giant mental slog because of how daunting each workout felt.
That said, these intervals require a ton of fuel. If you are not adequately fueling for the work (both on the bike and off) it makes the RPE a lot higher. Those long sweetspot/threshold days burn a lot of kcal in particular and that hunger can stick with you to the next day, too.
Biggest thing I ever did for improving my workouts was improving my diet and eating more, specifically eating more carbs. During my last SSBII pass, it was a million times easier to get through because of better nutrition. I cannot stress enough how important it is for this portion of the plan in particular.