A lot of supers could punish Lobo in his 11(gap)3 and b1(gap)2, not just Superman's. His super hits the most quickly, but (all?) supers have frame 1 armor/invuln. Any other moves with frame 1 armor or invuln can go through the gap as well, though like the supers, they are not necessarily fast enough to punish. Batman/Killer Frost/Wonder Woman's parries can punish them, too (Couldn't get Joker's to work, so either the gaps are both one frame, or my execution was off).
b12 is +4 on block with a small gap (1 frame?) between the two hits. It's probably not a good idea to high shot after this string since the opponent will be likely be blocking low anyway and the opponent can't even move yet since there's too much blockstun on the 2, so even if they try to do something, the shot will whiff over them. If they stand-block the 2, it's gapless, but you're still sacrificing +4 and pushing the opponent further back whether it's blocked or not.
21 xx highshot is fine, I think, since the opponent either stands to block 1 or gets hit and forced to stand anyway. I generally try to use the 21 string only when I have nuclear shells loaded, but early on I experimented with 21 xx highshot and 21 xx grab as a mix-up and found it too unrewarding since high shot on hit is +1 with pushback (too much to reach with any normals) and on block is -1, so whether it hits or not, it's hard to continue pressure. There is also a small gap between the 1 and the highshot that can be armored/backdashed, which the opponent may try anyway to avoid 21 xx grab. 21 xx lowshot is the only gapless pump shot off 2 or 21 and it's -9 without trait.
One thing you can do to make strings safe as well (like 21 and b2u1) is MB Hook Charge. It costs a meter, but leaves you + on block (this amount varies, but I believe it's always +), and leaves you close enough to do follow-up pressure. b2u1 xx hookcharge has a 2-frame gap and 21 xx hookcharge is gapless. I've been using b2u1 xx MB bf3 to make b2u1 safe against people who try to jump out of b2u1 xx grab and I think it works well.
EDIT: b2u1 xx lowshot has a gap, my mistake.
DOUBLE EDIT: ...As does b2u1 xx hook charge(man, I suck at testing things properly).
