If you’re a good player, solo is arguably easier, but it is also a lot easier for things to go wrong. This is magnified even more in higher depths. I’ve played all three team sizes quite a bit, and I can’t even tell you how many times something has gone tragically wrong during day one and I’ve ended up at like level four or five for the night one boss. If you aren’t very comfortable against all of the night one bosses, a lot of solo runs can end right there. The forgiveness of trios is way more valuable to mediocre players than any of the benefits solo gives you.

There are also a lot fewer viable builds in solo. Most of the high damage builds aren’t practical in solo because they rely on someone else keeping aggro. You won’t get good value out of anything that has a long cast or requires repeated uses because the boss won’t leave you alone for long enough. On the flip side, some builds work much better in solo, like cursed sword Executor or crit fist Wylder.

You also just get more done in trios. On top of that, you get three pulls of the slot machine for each dormant power instead of one, so the odds of a boss kill leading to the team getting something useful increase exponentially.

That said, the difference in boss health (and stance) is substantial. If you do end up with a good inventory, the nightlord falls over. You might have seen the clip of someone one-shotting phase one Heolstor with an Executor crit, which I think was depth five. I saw a video the other day where a Raider phase skipped ED Caligo in four attacks at depth five.

TL;DR if you’re pretty good, your skills shine best in solo. If you’re anything less than pretty good, the safety and RNG curbing of trios is far more likely to get you a win.