Lots of good options and successful uses for a variety of carts!
Stringy probably puts more miles on his carts in a year than most of us will in a lifetime. The fact that he uses a scupper cart is a pretty strong endorsement and I agree 100%. My scupper carts have been hauling Hobies since 2006 and I have never had a cart or boat fail in transport. I use only the Hobie standard cart, but it has hauled every boat in the Hobie fleet including TIs (probably not recommended) and Pro Anglers (relatively short distances).
I have a couple of different strap-on carts for non-Hobies, but IMO they don't compare -- vary rarely use them.
The foam filled Hobie wheels are impervious to flats. The cart is easy to insert and extract in the water, stows on the boat, floats, weighs little and is the quickest and easiest piece of gear that I know for launching and recovering. The carts work well on rutty roads, small rocks and packed sand. Definitely difficult in soft sand, yet I even use them there for infrequent short crossings.
IMO, most boat damage that has resulted from scupper carts has been from mis-use -- dropping the boat or putting weight on a partially inserted cart (cart legs not extending completely through the hull). and hopping curbs. Now that all new boats have scupper liners, it would take a lot of talent to break a boat with a cart! I use mine with 100% confidence, even on the older boats.
If stringy recommends the heavy duty Hobie cart for extended use, that's what I would use.
I would tow with a bike up to about 5 MPH as long as the road is relatively smooth (and you have a place to secure the bike at the launch point). It should be no more difficult than towing a kiddie cart. If the rolling surface is rough (i.e. any longitudinal strain against the hull), I'd get off and walk it. Common sense should prevail!