GHB Character Merging

That is entirely up to you. If you really like the unit, I say go for it. I've max merged my MKana, and I regret nothing on doing so. He's a solid member of my dragon team, and can outperform Nowi on a lot of occasions.

So if you like Robin, and have a great build for her, go for it. :)

It depends, in Robin's case i'd say that it's not worth it as you have someone with similar statline as her in the 3*/4* summoning pool with Boey, and she is the only one with B tomebreaker.
If she is your favorite then is worth

If you really like/use the character go for it but if not it might just be a "waste" of feathers Units that are summonabe will allways come out on top with +10 availability

It's up to you dude. It really depends on how committed you are for that character. I mean if you really want a 5* Merged Female Robin then yeah, go for it. I've seen good builds for her on this site so do feel creative if you do. Ive done a max Merge for my Arvis but after that, I've felt more committed to work on a unit that can get +10 easily.

When it comes to GHB units, there are several factors that require consideration.

1. If you prioritize high tier arena, you may never be able to reach high merge levels on the unit.
2. Some GHB units have unique skills that you may prefer to inherit to a unit(s) that can be max merged more readily.
3. Some GHB units may fill a role better than any other unit you have available.
4. Maybe you just really like the character and want to use them.