Flames are safe on shield. You can't abuse that in 3.6 as much as you could in previous versions due to the removal of flame cancel, but the flames themselves are stronger than ever. Spacing Fair on shield can be hard to punish for many characters even though it's not positive on shield. If you're outside their grab range after the Fair, you're taking away many characters' fastest option out of shield (frame 7-8).
In the case mentioned above of autocancelled Nair on shield, your opponent has 2 frames to act out of shield before you do out of landing as Boiko explained. This doesn't necessarily mean they can punish you, however, even if you're not spaced outside their grab range. Assuming both you and your opponent are frame perfect, they will start their grab 2 frames before you can act. Bowser's fastest option out of shield is UpB, which grants intangibility from frames 1-4 then a hitbox on frame 5. UpB's hitbox would come out 7 frames after your opponent has grabbed, which will unfortunately result in you getting grabbed, assuming a frame 7 grab (Falcon and Bowser have slower grabs, among others). They would take damage from UpB but you would lose the exchange in this case. To win this sort of interaction, you would actually have to wait 2-4 frames after your autocancel landing lag to UpB to make use of the intangibility. This way, you are intangible during the 2 active frames of their grab, then the UpB hitbox comes out and hits them. Application of this is considerably hard to do consistently and intentionally, but you should, according to probability, win a majority of exchanges like this. More specifically, there are three different frames on which you can UpB and win the exchange, while there are only two frames on which you can UpB and lose the exchange, ignoring all timings past frame 5 after your landing lag is over. I'll attempt a visualization of this since explaining it in adequate detail is a complete mess with just text. The three following scenarios assume this is after a frame-perfect autocancelled Nair on shield (-2 frame advantage). Apologies for lazy formatting.
I've bolded the most important frames in each table. As you can see at Frames 7-8, Bowser's intangibility frames (i-frames) from UpB run out right before the opponent's grab comes out. Therefore, Bowser is grabbed and the opponent suffers 13% from the first hitbox of UpB (this situation is often referred to as "grab armor").
In this case, the last frame of Bowser's intangibility and the first frame of the opponent's grab hitbox overlap. However, the result of this interaction is the same as above due to the second frame of the opponent's grab hitbox not being avoided by intangibility (Frame 9 in table).
Here, Bowser wins the exchange because BOTH of the opponent's grab frames overlap Bowser's i-frames. UpB's hitbox comes out 1 frame later to punish the opponent's endlag. This result also occurs if Bowser waits 3 frames and 4 frames after his landing lag ends. After these 3 frames, however, the opponent will win the exchange again due to his first active grab hitbox coming out before Bowser starts his UpB. Hopefully this demonstration was more clear than the abomination of a paragraph I wrote above.
Jab1 is also notable for the same reason as AC Nair: most characters can't punish it if spaced correctly on shield. At low % the opponent can crouch cancel Jab1 for a punish, and you should be wary of them Shield DI'ing towards you, but otherwise it's fairly safe.
Edited for clarity and accuracy.
