There are many myths and legends surrounding cast bullets.
First: BULLET FIT is King! With a properly-fitting cast bullet (meaning, correctly-sized FOR THAT FIREARM), most potential problems never occur. The standard "accepted" diameters for JACKETED bullets are often too small for good cast bullet performance, and often result in leading.... far more often than a case of just having a "too soft" alloy. For example, most of my cast bullets for nominal .308" bores are sized at .311"...and they work superbly, with great accuracy and NO LEADING.
I've made something of an amateur career out of testing-and-trying various things that "everybody knows" in handloading, with the aim of proving -or disproving- the concept in question.
One of these tests involved PURE LEAD bullets in my .416 Rigby rifle. I loaded fifteen rounds with the RCBS 416-350 bullet cast from pure lead, using the same charge as used in my "standard (cast) load" in that rifle.
With bullets cast ffrom WHEELWEIGHT alloy, the rifle groups TEN rounds in about the magic one inch at 100 yards, departing at 2050 fps. With the PURE LEAD bullets, the ten-round group was 1.25".... from that muzzle velocity of 2050 fps. That would be a marvelously effective hunting load, believe me.
After firing all fifteen rounds, there were NO deposits of lead in the rifle's bore.... NONE, nada, zip, rien. The inside shone like a mirror. In a rougher bore, or a worn one, perhaps there might have been some visible lead fouling. In THIS barrel, there was none.
The fact that something 'sounds reasonable', doesn't necessarily mean that it's correct.
Even so, I'd lean to an alloy that's a tad harder than pure lead, mostly because some folks are uncomfortable with the idea. Regular, unhardened lead wheelweight bullets generally seem to give decent results on game, and some limited expansion can usually be expected. HARD hunting bullets are a no-no in my book, due to relative ineffecteiveness.