I'm more than happy to be in the 'most of them are doping' camp but the motors thing doesn't ring true....think about the logistics.
Doping is easy enough to facilitate as an individual, Armstrong used to take himself off to Ferrari's place, Froome disappears back to Africa by himself often enough to raise questions but motors require a conspiracy and agreement, not just asking a team but across rival teams and hungry rival riders.
We had an expression in my previous field of work:
Tell nobody and nobody knows, tell somebody and everyone knows.
In other words doing something nefarious by yourself and keeping it secret is easy enough to do but the minute you tell a trusted colleague or a best friend they also then tell their best friend or a trusted colleague until leaks appear and in the end the truth comes out.
Motors in the bikes would require mechanics to be in on the gig, the data/analyst guys to be in on in too, maybe the UCI, then teammates and even if a rider and his spanner monkey kept it between themselves the data guys would be staring blankly at their screens trying to work out why X-rider's bike spikes in power by 50 watts (or whatever) on each haute categorie climb, and then the offending rider has to make up reasons for his improvement that other riders will buy and his mechanic has to start acting very suspiciously so the other mechanics don't spot him stripping down a motor, and egos have to managed as rider-Z may have thought he was heir apparent to be the next team leader and GC contender but rider-X is now blowing everyone away, total team collusion in other words, and what happens when rider-Z (annoyed at not being leading the GC charge) swaps teams?...does he take his suspicions to his new team and tell them they will have to use motors to beat rider-X...that's now two teams in on it and so on and so forth until I suppose the whole peleton are in on it...with the associated mechanics, data guys etc...that's a he'll of a secret to keep...and during all this no single rider thinks to drop a rival in it like Armstrong is rumored to have done with his rivals, guys accustomed to winning just sit quietly and watch their careers get stolen from them?...at least with conventional doping in a perverse and bizarre way they all had access to EPO, steroids, HGH etc (the internet is a wonderful thing) if they decided to go down that route, what about the riders who are lacking charisma and aren't well liked in the sport?...are they supposed to have the ability to suddenly change character and conduct an elaborate mechanized doping program that they trust nobody will speak about or are they the ones shut out on the fringes while the rest of the riders enjoy the newfound motor doping?...they seen ideally placed to speak out and level the playing field back in their favor...who decides which rider will be in-form this year and get a pass to motor dope, do they all meet in secret at the beginning of the season and draw straws to divide up the Belgian classics to this rider, the GC tours to this other rider and the second half of the season classics to yet another rider...seems far fetched and again you're relying on ego maniac riders that wantv to win just rolling over and agreeing that Sagan can have it all this season but Kwiat must get a chance next and for 2018?....sorry human nature (and especially the nature off elite sportsmen) doesn't work like communism....and in amongst all this are reporters and journos desperate for a scoop, motorized doping would be a big one (probably the biggest of all, bigger than Lance) but I suppose they're in on it too and are happy to let the biggest story of their careers go by?...again human nature doesn't work like that, one story and one book with the required T.V. appearances after and they're set for life....or they keep quiet and accept an annual wage, the endless traveling and the lack of job security as newer younger journalists come up or the newspaper they write for goes bust...for motorized doping to be viable pretty much the entire sport and those reporting on it have to be in unison and agreement, no dissenting voices, all teams and riders have to be prepared to put aside personal rivalries and work in agreement as to who has motors and who doesn't and if the simple answer is that they all have them they why no pics from paparazzi, reporters, fans etc while snooping around the mechanics and the bikes... I know I'd happily post online (or sell) pics I had taken of a pro cyclists bike with a motor embedded in it's
If one gets detected at an elite level then I'll happily say I was wrong but for this to be 'a thing' that's regularly occurring at a world level too many people have to be involved for it to work smoothly and those that know would then have to be happy setting aside all personal ambition to allow order riders from their team (and other teams!) to take the spoils, I just don't buy it yet...it would take a culture change in both the sport and human nature in general to work.