6B only gives the shakeable stun on counterhit; so if you find yourself in that situation you could just go bA for the ninja rush which does quite a bit of guaranteed damage (on CH). If you are using 6B for the reach, you could use 4K which does quite a bit of damage also (doesn't need counter hit but it's a bit slower). If you want to stick with 6B, then don't mix up with 214A. This move is pretty slow; they can shake and block on reaction. For that mix up and in general when you have the frames to do a mixup, I would do 33B or 4K for the mid part, and grab/iFC 3K for the other.
Yoshi has a lot of stuff that works well against people that don't know him. Med is a perfect example. The B is easily steppable on reaction, the K is easily anything-able, and given that the other two options are so easy to stop there's not much room for the A to hit. If you use Med once the other player will probably be content with just blocking or maybe eating Med A. But if you use it more than that, against a good player you will either whiff, get GIed, or get your low blocked. I.e. you are probably eating a launcher. However, if they don't know your options well then they can't shut you down like this.
Reflection (his backward facing stance) and intimidation fall into this category as well for the most part. I never uses any of those three stances except when messing around against people who are very bad. Flea can be quite useful though; you have good evasive options (very fast step, constant tech jump, tech crouches and leads to frame traps). You can also play wake up games with DGF and SDGF, or even just mix up games if you are feeling brave. These are for the most part technically unsound; from a given stance there's a way to defend against all options without guessing (i.e. no real mixup). On the other hand, to be honest you need quite a high level player to execute this in a real game. If you know all of his stances and options well, you can switch very fast and not give people a chance to adapt.
The reason I quit Yoshi is because I couldn't execute iMCF and couldn't be bothered to learn, and its an important part of his game. It's quite hard. You can still play Yoshi well without it though.
To see someone play a pretty crazy Yoshi at a high level without a ton of iMCF, check out videos of NeoRussell especially older ones where he doesn't iMCF as much. If you want to see a less crazy, but very very lethal Yoshi then check HajimeOwari. He doesn't have as much crazy stance stuff, although he does use the Tekken moves amazingly well (another reason I quit Yoshi, concentrating on the game + the timer was too much).
