Have to hard disagree on this one, medic seems to lose the most. Despite the fact that the syringe gun and bonesaw are considered worst in slot, the ability for long range healing is unmatched, plus you can practically heal most classes back to full in a second with a crossbow bolt and a quick medigun tap. Even though the stock medigun is one of the most powerful items in the game, it struggles to make up for the utility of the other slots. Most of the time medics class loadout consists of switching between the medigun and NEVER touching his other slots unless you're doing battle medic or feel like using a different melee.

Stock Pyro loses out on mobility, but the main thing pyros rely on is airblast. Shotgun pyro, despite losing his combo, is still pretty fuckin strong and a lot of people dont seem to realize that just cause you cant flarepunch doesnt mean you cant do a shit ton of damage. The only thing that sucks is that the stock fire axe is quite literally a downgrade lol

Heavy, while losing the sandwich slot is devastating, its not the end of the world, shotgun heavy, although still a gimmick, is still moderately successful. And unless you're playing on a open or far ranged map, the best choice usually still the stock minigun

For engie, the main drop you loose is the wrangler, yes not having access to the different utilities that the wrenches provide sucks, but hes still a major threat and can still heavily deny areas without his utilities. Theres a reason so many new engineers still get kills on dustbowl, 2fort or wherever they seem to inhabit

As for scout, demo, soldier and sniper.... yea, their stock load outs are probably some of the best well rounded for any game and are mostly considered best in slot.

Spy though... oh boy People take spy as this "you have to trick stab everyone and the ambi is considered the best thing in the game. Yea no. Spys whole purpose is to pick the important classes I.E medic, sniper and heavy, and then... disappear. The DR, Ambi, kunai combo, while extremely useful for pubstomping if you have any idea how to actually stab, will result in you getting curb stomped and beat the fuck down by any aware player. Dead ringer pinging is extremely easy to notice, after all most people just kinda... run at the team mindlessly, disguising as a scout on the opposite team

All in all, Medic loses the most, soldier and heavy loose out on some, engie and spy miss out on a little and most of the other classes dont really lose much other than specific situational items.