Lol Samus' CT on Pichu is kind of disgusting, I used it a bit against Chu in my MMs against him.
Hylian has it mostly right though: that if your foe just DI's straight away, they get too far away, but there are some characters for certain percents where you can perform a 2nd or even 3rd CT. Ganondorf, from about 30-50%, you can CT. Pichu works for under like 150% lol. The tricky part about CT'ing with Samus though is you need to quickly respond or predict their DI: If they DI away from you, you have to dash-grab. If they don't DI at all, you standing grab, and if they DI behind you, you need to usually dash forward, DD back then grab all very quickly. You have to do these things to get your body to where they are; as pointed out, the grapple beam cannot grab in the air. I've also gotten 2-3 Samus CT's off on Ness, and a bunch before on Yoshi's. They need to have a good bounce stun from your down-throw, if they have that, then you can do it at certain percents, and if you predict their DI correctly. So to sum up, it can work against at least: Pichu, Ness, Yoshi, Ganon (these are the characters I have used it on).