Good sonics spindash into up-b which makes him extremely difficult to punish unless your character is very fast in the air.
Which require a jump to perform hence lose his "invincibility".
Anyways, I went to NinjaLink and his crew to test something about Sonic's Burning Spin Dash. It is not invincible... it is much much worse. Burning Spin Dash doesn't have a Hurtbox. My pal Liege test this out with Marth's FS. Critical Attack goes right through Burning Spin Dash. We concluded it might be a coding issue like Diddy's Uair, Ultimate Shulk, or Fire Kirby. The second sonic jumps, stops or turns, his hurtbox returns. So maybe this was intended. With that being stated...
I Tested with my brother, I found few things to counter to Burning Spin Dash:
1) Counter - in order for Sonic to regain hurtbox he needs to hit something. Counter moves will allow you to tank the hit and punish Sonic's recovery lag. Mii Swordfighter, Mac, Ike, Greninja, Lucario, and Palutena all have good results for consistently countering BSD. Best ones are Mac, Greninja and Lucario. Marth/Lucina can stop BSD but they counter too quickly to punish sonic. Shulk's vision slow the gameplay down which failed to revert Sonic back to neutral hence whiff him completely. If you going to use Shulk's Vision, make sure to take Power Vision and use the normal slash version. It is the most consistent out of the 6 (normal and forward).
2) Shield - As you already know, you can just shield BSD. This is common sense and imo the best option. Unless Sonic decides to Jump and Up B, you can punish his recovery lag. If you see him jump, you jump cancel out of shield and trade with an aerial.
3) Spot Dodge - BSD only loses his "hurtbox" when he is burning. But at certain distance (3/4 of FD) or when Sonic is turning his Hurtbox returns. If you spotdodge and attack behind you, you will hit Sonic out of BSD. I perform this with Lucina.
4) Mid Jump Aerial. You jump high enough to evade Grounded BSD but low enough to contest Jumping Spin Dash.
5) Distance - As mention in number 3) at certain distance (3/4 of FD) Sonic Hurtbox returns. It was weird how I got this out. I was spamming Gale Strike, Swordfighter's Tornado move, and I manage to hit my brother out of BSD. I was shocked. No way that Swordfighter can beat this "op" move. After some more testing we later realized BSD charge/speed determines how long Sonic is untouchable.
*) Super Armor does not work. Sonic goes right through you.
**) Two BSD against each other result in a clash. But if one of them jumps, the Jumping one takes damage.
New Information -
Burning Spin Dash and its code are just weird. I spent a hour labbing with my friend Liegemaximo and we discover some janky stuff. 1) BSD does have a hurtbox, but I believe it is priority based. Here how we came to this conclusion -
6wx is the only player we both know that use BSD in a tournament. However, he used it on certain matchups. Question here is why? If the move is superior and I guess broken, 6wx should use this move more often. He didn't even use it on Dabuz's Rosalina of all characters. Well... apparently, Rosy or better yet Luma can stop BSD just by Jabbing. No, I am serious!
We ask ourselves... how is this possible? Every time I dash into Luma's constant jab clash sparks appear until I was finally hit out of BSD. After several tries and being mindblown time, I conclude we need to use another character. I know WFT has a super armor explosive move and a multiple hit projectile. BSD should be able beat both right? Hahaha nope! Fully Charged Sweeping Sun Salutation, if shot before, will beat a Fully Charged BSD every single time.
Oh and Volatile Breathing... it was literally WHAT moment. I have never seen an explosion clash out before but this take the cake. YOU ALL NEED TO TRY THIS. Violated Breathing with BSD put both Sonic and WFT into neutral. There is no Explosion! Some how, the great Sonic absorbed it.
tl:dr - Burning Spin Dash is not "broken" nor invincible. Go into the Lab and test it yourself. Do Smart Bomb vs BSD, Marth's FS vs BSD, then move to Little Mac's Counter vs BSD, Luma's Constant Jab vs BSD, WFT Sweeping Sun Salutation and finally Volatile Breathing. Nintendo need to work on its coding. I ask Liegemaximo to get NinjaLink to do a video about our findings. We will post it ASAP. Until then... what is our next broken move?