Power Levels are okay, but can break down quickly when you factor in unit upgrades as these are basically "free".
So if you and your opponent both have 7E armies and want to play a game with together with your models and units as-is in 8E, then the chances of getting reasonable balance are decent. This is because under the previous system, you are both likely to have had some upgrades on your units, but points would stop you from going wild with them.
On the other hand, if one of you has a legacy army, and the other builds an army specifically to leverage PL in 8E, then you might well find the game is rather less well balanced if you use PL.
Think of the famous battle company formation that SM players had in 7E which gave everybody free transports, and how those 10+ free transports basically meant getting a free extra bonus of about 20-25% of your army total over what your opponent had. ...Not very balanced, was it? Well, PL can be the same if one player decides to abuse it deliberately.
So basically, if you and your group normally adhere to Wheaton's Law, then using PL to get a force together for a game is likely to be fine. Okay, if everyone has been fair and restrained, the difference in points between two armies with the same PL might still be in the 5-10% range, but that's much more manageable and balanced in practice than a difference of 20-25% is.
If you have the models, you may as well talk to you friends and give it a go - you don't really have anything to lose.
Edited by Major_Gilbear, 29 November 2017 - 02:12 PM.