My point is that Lightning has additional failure modes that BTC does not, and Lightning in itself does not offer the guarantees that Bitcoin does. It of course also suffers from all of BTC's failure points - if someone successfully does a 51% attack on BTC, they can implicitly also steal any Lightning funds as well. If you close a Lightning channel and then don't wait for enough confirmations, or you broadcast your cheating transaction and don't wait for enough confirmations, you can clearly lose your money.

The risk doesn't increase with the number of transactions on a channel, that was a wrong statement from my side. What I was thinking of is that the risk increases the more your transact through Lightning instead of regular BTC. Basically, the more of your BTC is caught up in Lightning channels, the higher the value of attacking you with a double spend attempt.