Nothing, other than the reputational hit they would take. There a level of trust, but we allow banks to hold our dollars. Just don't hold a lot in there, treat it like a real wallet, a few hundred bucks maybe, not thousands. 

 That’s very contrary to the principles of #bitcoin, defeats the whole purpose of bitcoin

#lightning is flawed 

 Yes, that is a flaw, but we cam run our own nodes and control it ourselves as intended. 

 Hypothetically, could a #lightning node/wallet be built into the #Bitcoin core app? 

Or would that be hard/compromise security somehow? 

 That's beyond my understanding or skill, others on Nostr might know.