I also play 1.Nc3 (both online and OTB) especially OTB in games that are relayed into the internet (to fool my possible opponents...)
The french and caro-kann setups aren't a problem, and neither is 1...e5 (which you didn't mention) The problem line is this:
I didn't mention 1...e5. Wow yeah I knew I was missing something else. 2.Nf3 is my move of choice there. I purposely didn't go with 2.e4 against 1...Nf6 because it gives black the power to transpose into the Vienna. There's nothing objectively wrong with it, but enters mainline 1.e4 territory. 4.e6 does look like it chokes black though. The open f-file and backward doubled d-pawns are easy targets.
But if black plays 2. ... e5 you either get a Vienna or a 4 knights in both openings white has no advantage
1)halloween gambit: problem solved!
2) 1.Nc3 d5 2.e4 Nf6 is possible too...
Pros and cons of 1.Nc3!
Pro: Develops a piece
Con: It's a queenside piece, doesn't contribute to kingside castling.
Pro: It's flexible
Con: It's flexible for black too.
Pro: It provokes black into making pawn moves, creating weaknesses in his camp.
Pro: White makes a minimum of pawn moves, so there are fewer weaknesses made.
So how did I do? I won't be offended if you say my analysis sucks since I've just started studying this. Where can I improve here? 1.Nc3 steers the game into territory that is flexible and minimally theoretical. Skill rather than memorization should decide, although positions may be similar enough in some lines where black recalls his strategic plans anyway, but taking into account that the e-pawn isn't committed yet (in the case of Open Sicilians with 1.Nc3,c5 2.Nf3,d6 3.d4)
I still didn't cover French or Caro-Kahn setups with this yet, but still need to think on those. The French especially looks tough to face as it supports a d-pawn, maybe 2.e3 after 1...e6?