The bootloader is a large part of it. Unless Motorola decides to start unlocking them, I wouldn't expect any major increase in the number of roms. I'm sure a few more will come along, but you're not going to see the type of developer support on the MAXX that the Nexus has. If roms are really important to you, you should definitely go with the Nexus. If having a phone that actually maintains reception and has great battery life is important to you, go with the MAXX. For me, it was a night and day switch coming from the Nexus to the MAXX. Sure flashing roms is fun, but I really need a phone that's actually going to work. The Nexus had far too many reception issues for me (I had four, and they all had the exact same problems). After going with the MAXX, I haven't looked back since.
Also, are you looking just at roms that are listed specifically for the MAXX or are you looking at RAZR roms in general? Roms for the RAZR will work with the MAXX as well since they're internally identical on software and hardware except for the battery.