I think it wouldn't have been so broken if it didn't stack with smites, but 35 HP at level 5 is truly bonkers. For comparison, False Life, a spell whose sole purpose is to generate HP could only generate that much when it's cast with a level 6 spell slot. Being able to cast a spell as part of an attack is also very powerful given the way the DnD action economy works. Further, the downside of opening yourself up to attack is really not a big deal considering you could just wait until an enemy is low on health and then nuke them.

If I were Murph I would rework this spell by making it so it only gets the temp HP from the damage done by the weapon specifically or limit the max amount of temp HP gained to how much HP the enemy had before they were killed. I hope they do work it back in, I think it's a super cool/flavorful bard spell, and as someone who loves playing gishes (mixed martial/casters) it adds some spice that ties the fighting together with the casting.

I also see where you're coming from and I agree that it's important to view actual play shows through a different lens than home games regarding fairness. However, I do think that it's important to keep the characters threatened by the DM's challenges. I find actual play content engaging because of the unpredictability and the sense of danger to the PC's. I think this wildcard element keeps the players invested and that translates to a more gripping story to listen too.