If you don't go too heavy with the dingy and motor the swimstep should hold the weight if it was a well-built swimstep to begin with. We have a 9' Livingston with a 4hp 4-stroke Yamaha on Weaver Snap Davits on our swimstep. The motor is on a swivel mount bolted to the Livingston's transom so it can be carried vertically when the dinghy is also in its vertical stowed position. We use the boom fall to deploy and retrieve the dinghy.
The slickest system for this arrangement is a Seawise davit that not only cranks the dinghy up onto its side but also automatically positions the dinghy motor in a vertical position. Very spendy, however.
Carrying the dingy horzontally is another good way to go and offers a few advantages over the swim-step Weaver-type davits. In rough water the dinghy is quite aways above the waves and is very unlikely to scoop in water which can happen with a vertically-carried dinghy on a swimstep, particularly if it's an inflatable/RIB because the water will get trapped inside the dingy--- lots of weight applied suddenly to the swim-step and its hardware.
The horizontal position means you can launch your dinghy in a big hurry in case of an emergency. You can with a vertially-carried dinghy in Weaver Davits too, but the horizontal systems are proabably faster. But then you have this big thing hanging off the rear of your boat. Whether that's a problem depends on where you take your boat, what your home slip is like, etc.
Unless you have a very wide swimstep, a dinghy carried either horizontally or vertically will most likely render the swim step useless when the dinghy is in its stowed position. If you typically use the swim step as an aid to get to the dock while docking, you may lose this ability with a dinghy back there. Also, if you rely on a folding or retractable ladder as part of your MOB recovery plan, there's a good chance a transom or swimstep mounted dinghy will render the ladder unusable when the dinghy is in its stowed position.
We won't carry a dinghy on a cabin top, either aft cabin, sundeck, or boat deck, for a variety of reasons. (We do carry a lovely little Montgomery sailing dinghy on the aft cabin top but that is not the boat we rely on for our everyday shoreboat use.) And our boat has pretty low freeboard aft so we don't need the swimstep during docking. So our vertical dinghy stowage with Weaver Davits on the swimstep has worked out for us very well. But when we start taking longer cruises we plan on getting a 10' Bullfrog and 15 hp motor, far too much weight for our 37 year old swimstep. So when that day comes, we'll tow the Bullfrog.