what ever you do, DO NOT put SPD MTB cleats on road shoes, this is a bad idea. The MTB cleats are meant to have the support of the side bars found on MTB shoes and are not designed for the flat sole of a road shoe and so they are not optimal and could lead to issues if you are looking at power and performance. But as said above, a really stiff MTB performance shoe should have similar lack of flex to a good road shoe. There are differences in the weight etc but again that is the sole and maybe at the high end of things they get closer together but a MTB shoe is typically heavier than the similar model in road.

what ever you do, DO NOT put SPD MTB cleats on road shoes, this is a bad idea. The MTB cleats are meant to have the support of the side bars found on MTB shoes and are not designed for the flat sole of a road shoe and so they are not optimal and could lead to issues if you are looking at power and performance. But as said above, a really stiff MTB performance shoe should have similar lack of flex to a good road shoe. There are differences in the weight etc but again that is the sole and maybe at the high end of things they get closer together but a MTB shoe is typically heavier than the similar model in road.

Perhaps I don't understand what you mean by "side bars"

Could you explain some more or provide a picture?

Hi I tried to post a picture of it but it did not seem to upload ... anyhow if you look at say the Giro empire mtb shoe on their site, and then look at the sole there is a raised rubber section that runs along the length of the area where you would attach the cleat to the shoe (along the outer edge of the fore foot of the shoe). These are on either side of the cleat. These are the bars I am referring to. Road shoes are a simple flat sole.

The idea that it’s getting dangerous in transition because of slipping has me thinking about mtb shoes also. SPDSL cleats have small rubber pads that are insufficient for any traction, Look cleats are basically ice skates. These cleats are designed for cyclists, not triathletes running with a bike in transition.
Don’t know why companies haven’t added a Tri/road cycling shoe with spd recessed cleats.

Hi I tried to post a picture of it but it did not seem to upload ... anyhow if you look at say the Giro empire mtb shoe on their site, and then look at the sole there is a raised rubber section that runs along the length of the area where you would attach the cleat to the shoe (along the outer edge of the fore foot of the shoe). These are on either side of the cleat. These are the bars I am referring to. Road shoes are a simple flat sole.

just a note to keep things straight, SPD just stands for Shimano Pedaling Dynamics, so it really references both road and MTB.

If manufacturers made a lightweight, (non leather) MB shoe that didn't cost the earth I would buy them in an instant.

I second the answer, none.

Having looked at a few tests on GCN and other places then I tried, for a lark racing Tri in MB shoes with spd's.

Bear in mind that my power output might at best be half of a true road bikers capability.

MB shoes are easier to walk/run in, so mounting and dismounting is a breeze.
I have always put my shoes on in T1, and Race mounts are faster, because you can do a one pedal, leg over, without slipping.
I regularly run past a bunch of people trying to get mounted or trying to get their feet in after starting clipped in.
In T2 I can stop on the line and hop off while still moving. And don't have to spend the last K untying, taking my feet out and coasting.

sdp pedals are usually 2 sided, so you might even gain a few more seconds clipping in..
I also find they unclip better/faster, so the embarrassing "topple" doesn't happen so often.

Power loss is minimal (if any) I have tried on matching courses to find a difference.

And another bonus is if you have a problem (like a mechanical you can't fix) on an isolated ride you can walk (even jog) on MB shoes.
If manufacturers made a lightweight, (non leather) MB shoe that didn't cost the earth I would buy them in an instant.