Also not sure if it matters to you, but the wired track SRM's are not waterproof - not sure about wireless track units as happy with my wired units...
For accuracy and repeatability of data, it is very easy to check the slope of the SRM units yourself with a known weight and then update the crank via the SRM software or PCV. Interestingly PowerMax claim this is unnecessary.
I wouldn't be put off by the battery not being readily replaceable as it lasts a number of years even with regular use.
Note I own two wired track SRM's and a mix of wired and wireless road units too, so if you have any specific questions then fire away.