The problem with on chain is that it is unpredictable. If you need to make a fast transaction, you should avoid on chain and use lightning. If you happen to have an unlucky day, you could spend 1 or even 1 and a half hours waiting for the first confirmation. Whereas with LN either it works and a route to the other side can be found, or it does not work perhaps due to liquidity requirements if you are doing larger transactions. However, you will find within seconds or at most a minute whether it worked or not.
I personally never had any issues with reasonably sized transactions on LN. It is perfect for when you have to make a fast transactions of low or medium value.
Transaction on-chain can be predictable if only you understand how miners incentives transaction. Why do you even need to use lightening network if you are going to be doing a one time transaction. Using a channel, you are going to open a channel which need an onchain transaction fee. Instead of doing that, why not pay the fee you are avoiding and have your Bitcoin confirm within some minutes. If you pay good fee, you should get your transaction confirmed in 10 minutes on average.
It's even possible that you will want to open a channel but will take time, if you are not sending transaction like sending your coin through lightening to exchange like Kraken, you don't need lightening. If you are considering paying a merchant, you need also to open a channel with them and that will cost you another transaction fee, to close the channel you need to also pay some fee. Why not do your transaction through onchain and be done for once, transaction fee are even cheap now.