I tested heavy to mid multiple times when the buffalo steak sandvich was unbanned. I would say it was effect for about half the maps and in general for mid fights. For the maps it would be a problem with certain maps being too open. Examples of this are sunshine and process. There aren't easy spots for the medic to position himself around the heavy if a mid fight goes bad. Heavy can only do well if he isn't easily spotted on mid, meaning they can just slowly kill off people on mid 1 by 1 before they can get to a arrow tanked heavy, making the mid a L from the start. If you run it with two scouts and 1 soldier to mid, your only saving grace is if your heavy is fast enough to get into a good position and will get arrow tanked while going across mid and your remaining soldier makes enough space for your team before getting eviscerated. I guess for sunshine, a thing a heavy can do is go to cafe fast and hide there before the opposing team notices and peaks when both teams meet on mid but then again it has to be tested.
However on other maps it can work wonders, especially as a surprise tactic. On snakewater I like it because the heavy can fast rollout to catwalk and stay there if soldiers fast bomb, making it easier for your pocket scout to kill soldiers, and your medic can stay lower and bait if needed if the team rolls out through cat. If soldiers don't bomb then your heavy can just get arrow tanked if he crosses mid and plays as a second pocket scout. Obviously this can work only if your team knows how to play with this and has trust in the idea of you running heavy to mid in the first place. On koth maps it can also work. Heavy can work well as a soldier denier if he plays cliff on product, or stay in shack on clearcut until your demo does enough damage and gets arrow tanked and peaks into point. In general, heavy to mid can work but it's very situational and map based. It can also be predictable against certain teams and may work against you if they know how to properly focus call. For me I think it works best as a surprise maneuver or if 0 mid fights are working in your favor, but this is just my opinion in the end from my experience testing it.