I have just put up an Ametek TDM with 4 PVC blades. The blade length is 45 inches. The setup is sitting on a 4 foot 1.25 inch diameter pole. Its attached to the gable with an Eave Mount. The Eave Mount has 2 brackets, 1 is 30 inches in length, it is in general designed to support a TV antenna.
Several issues should be addressed:
First: Strength. Eave mounts are intended to support an antenna against the antenna's wind load. A mill produces a HECK of a lot more wind load. In your case that wind load is at the 4-foot end of a lever. The other end of the lever is the separation between the upper and lower mounts. If that's, say, two feet, you've just doubled the force trying to pull the lower mount's screws out of the wall.
Another strength issue is the pole itself. The bending moment in the region of the upper mount is enormous. Too much load and it folds there. Result is your mill (which is probably overspeeding, too, since it just got hit by a gust and hasn't had a chance to feather yet) comes down on the roof or side of the house with the blades at wind load speed times the TSR.
Say your TSR is 6 and your wind is 100 MPH: Your house gets hit with a pvc wedge traveling at nearly the speed of sound. NOT good for the shingles - or the underlay, or the roof joists. (And it's probably raining REAL hard, or will be in a couple minutes.) That's assuming it hits the roof, of course. If it hits a window you could have the blade snap off against the frame and travel across the room at maybe 400 MPH, accompanies by the window glass.
Guy wires take this bending load off the pole, substituting a smaller one higher up. Do it right and your main storm worry is a blade coming off when it's traveling toward the house. (That's why I'd use a tower rather than an eaves mount. But then I tend to live in places with violent winds from time to time.)
I'd expect a PVC mill to be much less of a hazard than a wooden one in these situations. Less tendency to snap, lower density material so it loses a lot more momentum when hitting the house. (And with a commercial design rather than homebrew perhaps the designers did some serious multi-head design work on failure mechanisms. Maybe.)
Last is vibration: Mounting a mill on the eaves means any vibration it generates is transferred to your walls. This can make annoying sounds. But a bigger issue is that it can work nails and screws loose over time. Working nails loose in the house leads to leaks and damage - sometimes in places where it's expensive to fix. Working the mount fasteners loose means that somewhere down the road one of the mounts is likely to pull loose in high winds, leading to scenarios like those above (but with less energy in the blades.)
