If you're trying to break someone's egg when they have this kit and their egg is full obsidian'd then it will be a pain to break it since they would just respawn over and over again with their spawn-protection and keep hitting you with their axe and you can't do anything about it.
If I tried to block myself in and break their egg, sometimes it would say "you can't build here" so I would have to find a better place to block myself in, and in the mean-time I would be getting crushed by an axe over and over again which may eventually kill me before blocking myself in, especially if I only had full leather, chain or maybe even iron for that matter... (Maybe not in OP mode) Also, after you blocked yourself in they would probably break the sandstone before you break the obsidian and continue hitting you with their axe (unless of course you have an eff6/8 dia pick from OP mode).
If this was team eggwars with all of the people using this kit then breaking their egg would be a nightmare. From past experience of trying to break a full obsidian'd egg on a team eggwars map, I know how annoying it is not being able to block yourself in because all 2-6 people keep running up to you, punching you, breaking your sandstone with you killing them one by one which causes them to respawn with their spawn protection creating a huge cycle. But If they all had this kit...
I would be dead a lot quicker :)
I just think this kit would be overpowered and annoying (I'm already tired of the gold eff1 pick, I prefer to not have another annoying one)
I think if you were to remove the wooden axe then this would sound like a great kit!