Kunai is a bad weapon and there are too many bad spies using it. Only excuse for using it is the high of using its novelty for fun.
SIR. You have insulted my honor. Kunai is best knife to use when DRing:
If you take more than 60 damage as spy with your cloak down, more often than not you're boned anyway (against decent players).
If you're using the dead ringer the low health vulnerability can be largely negated anyway.
The health buff after a stab lets you (if spotted) escape without major risk, engage others head on, or continue a chain-stab even under heavy fire (since you keep getting overheals). As long as you make the first stab, you're golden.
You can ignore healthpacks completely. Just find another back.
Backstabs extinguish you. Totally boss.
TL;DR <3 Kunai
Sorry buddy, health bonus is less than a soldiers total, and they also die near-instantly to everything. Especially in the point-blank range of maximum rampup needed for stabs. DR works just fine with other knives and more health, healthkits are generally next to metal pickups you want anyway. Only now you can be oneshot by everyone instead of two!
Yeah, as a primary spy player kunai isn't worth the effort. Any pyro worth his salt isn't going to let a spy stab freaking anything after they find one. They just corner spies with airblasts. If a pyro findes you, the first thing you should be doing is making the DR fake-kill and extinguish you while you run like a little girl.
At most, the only two knifes I end up using are the normal one and the Eternal Reward, the latter only being used with the Familiar Fez & L'Etranger to make use of the silent decloak ability.
Silent DR decloaks are fantastic, by the way. Only downside is that because of the Eternal Reward you're basically worthless around engineers until you can stab up a decent disguise.
The nerf has really made sandviches much less useful. If you have a medic buddy, I understand, but otherwise, I don't see a reason to carry it around. The reason you use shotgun is that you don't get any movement penalty and don't have to wait for spinup etc. If you walk into an enemy group you have time to back off to cover and can punish pursuit at the same time Use shotgun while scouting and switch to mini once you determine where to ambush at.
I don't know why people think the sandwich is ruined forever. Let's ignore the fact that you're still a walking healthkit dispenser for your teammates (protip, not just pocket medics!). Clear and area but take a lot of damage in the process, get in an ambush position without full health, kill a pyro whose afterburn is going to finish you? Munching on Sandvich is still often quicker than running down a healthkit. Sure, sometimes you get caught munching, but honestly I think that makes it more balanced and fun. It still comes in more useful than the shotgun, for me.
The thing that made the sandvitch so useful for a heavy is that it made for a perfect retreat mechanic. If you were taking too many hits, turning tail, dropping one down, pulling out the running gloves, then retreating to the next choke point back was a fantastic strategy. Now, that mechanic is totally bungled because heavies now have to be incapacitated while eating it normally. Retreating under the buffer made by that initial, quick heal is made impossible now.