The way I see it, is if you are riding a multi-day event such as Indipac, (and all things being equal), a powermeter will be very useful, as you will be able to see how much power you are producing in conjuction with how your heart is performing, irrespective of how fatigued you are.
As an example, you could set a target of 150W and you could set a target to keep your heart rate in Zone 2 ... (you will know what power you are capable of if you train beforehand with the powermeter.)
the 2 sets of data will be better than just heart rate data. You could be seriously fatigued and your heartrate on it's own will be meaningless as you would 'think' that you are doing OK, yet you are not producing enough power due to tiredness.
If you set a target of 150W for IndyPac and maintained it, what would most likely happen is that you would win it!
Heart rate data isn't very helpful in multi-day events because the way that your heart responds changes with increased fatigue, ie the relationship between your power (or speed or any other non-biological metric) and heart rate would change by day, so you can't really use heart rate to measure anything.
TBH a power meter isn't a lot of use either. It can help you with pacing on the first day but, after 3 or 4 days, there is pretty much nothing that you can do with the information: you will be riding at the pace you can ride at for the number of hours you can keep it up.
