Mobile nodes exist for one reason and one reason only, to sell you something. Their entire model, and why there are so many of them, is the minimal friction via the app store to turn unsophisticated users into customers.

To the extent mobile nodes are self-custodial is a distinction without a difference, it's a distinction they use to shield themselves from regulators or to capture an audience for their LSP service.

Since mobile nodes are mostly offline, you have minimal protection since you must be on-line to defend any claims with the chain. If you're running a watchtower, you're running an always online service anyway and may as well have just run a real node.

In most cases, mobile node users are still using a completely trusted flow from the beginning anyway. Fake L2 wallets have taken this a step further and are completely Trustodial, while affinity scamming as self-custodial Lightning wallets.

Mobile nodes do not contribute to the network, as they are completely useless for the passive receiving and bi-directional liquidity which is a pre-requisite for any meaningful traction.

They're expensive since the footprint of a channel, and the liquidity enabling it, is not economical on a per-device level nor are any of those costs recuperated through organic bi-directional re-balancing. Those that use swaps from a Fake L2 are even more expensive.

All these achieve is a placebo effect for unsophisticated users, fee generation for the operators, and giving lightning skeptics plenty of fud ammunition because they work terribly.

Mobile nodes and "Node-less" (someone elses node) fake L2 wallets are an attack on Lightning. You don't run an email server or your businesses website on a phone, there's no reason to run a lightning node on one either.