Just like with on-chain fees. If the fee is too high, I'll try the transaction later, or from a different wallet / different channel. If the transaction is time-sensitive, I'm willing to pay most any fee. With LN, I'm very infrequently been surprised to the upside on fees. But I too would like to have a configurable default upper limit (in terms of sats per million) so that I'm not finding out after the fact that my transaction cost way more than I was thinking it would be.

I do like how withdrawal on SN permits me to set the max withdrawal (in absolute terms / sats).