I am not sure what platform do they use for trading? Which markets do they have? They are not very precise about it. Yes I heard about cases when people just buy via IB and transfer it out for safekeeping somewhere else (and vice versa for selling...). It is free on IB side︀ I heard to transfer positions out. With 5m+ holdings, you can also get Julius Baer︁ (CH) at 0.1% and they don't apply negative interest to cash balances above €1m. But︂ as we have already checked, Saxo for 1m+ investments have 0.05% custody fees (for MENA︃ region), while Internaxx has flat maximum €180, which just beats them all.
I would normally︄ use Standard Chartered Singapore for shares holding (no custody fees at all) and trading (0.2%︅ is acceptable I think), however similar to Saxo, they miss one position from my ETF︆ portfolio, which Internaxx and IB have, but out of those two I prefer Internaxx.
I always wonder how (apart from jurisdiction law) are these international group of banks like Paribas,︇ Citibank, HSBC dependent on each other's location. For example would US/FR/UK allow Citibank/Paribas/HSBC to go︈ bankrupt on one of their international markets? Or this is extremely unlikely.