But in my opinion it is not worth implementing as any other method then direct power measuring is very inaccurate unless the conditions are ideal (no wind, the same surface, still position on the bike...)
Of course innacurate on flat becouse of what you wrote :wind.,surface... but not so bad when we climb mountain( this is important for me).
Since a year I have also ''old' 'Sigma Rox 9.1 with Power Measurament without ANT+( just alghorytm) and it is very fine option in my opinion. For touristic cycling ( with a little sport bent) good enough. For ex. You climbed 10 km hill with avarage 150 watt, then after few week you try the same hill and you see avarage 200 watt .Clear it is a ,,progress''.
(I dont like IPBIKE becouse of ,, Maps'' and all navigation function. Locus is years a head in this topic).