High Initiative almost always is a disadvantage.

I have noticed this as a function of DM strategic behaviour more then a failing of the rules. DM's consistently use concealment on encounters to give opponents more time. The rules incentivize concealment for DM's. So high imitative players see nothing to target when the fight starts. They have to hold an action to get any chance in the first round. Opponents reveal themselves on their action, and those with lower imitative rolls get full actions against opponents. Making lower imitative of greater advantage in this increasingly common combat situation. Of course i expect others to post below that this is not the case, but being an observer of playing 1st edition and then 5.5 edition and no editions in between its a huge difference and a significant oversight be Wizards of the Coast. 1st ed was resolved like this.

• Determine surprise.

• Determine encounter distance.

• Roll initiative if neither side was surprised, or both were equally surprised.

• Resolve actions.

• Reroll initiative for subsequent combat rounds.

The problem is with the DM more then the rules.  I think it has a lot to do with real world inexperience on how actual ambushes work or don't work.  There are many ways to have ambushes found out in the seconds before they are sprung.  I would think that the high iniatative is what allows for that notice of an ambush.

Yes being Surprised by enemies ambushing you isn't ideal if you roll higher Initiative against those who Hide, but there's still things you can do such as;

• Search for hidden enemies
• Ready to act in response to a trigger you define
• Dodge to help protect yourself
• Help an ally in some other action.
• Dash or Move away to a more defensive position for example.
• Any other action you choose to do instead

The revision on Surprise rules may be a bummer to some, they're still more permissive than 5E before, where you couldn't act or react until after your first turn.

I don't think the typical play experience consists of most every fight starting with the players being ambushed.

I don't know that DMs do anything consistently. :)

For some characters, in some situations, high initiative is inconvenient, especially if what you do is melee attack and you're not clear that there will be a fight just yet.

But more often, getting to go sooner is a huge advantage because you can move or take out enemies before they do the same. Enemies are more likely to be nicely grouped for AOEs, you have the option to become un-grouped for their AOEs, any of your support characters can take actions to incapacitate the enemy or buff the party before the enemy acts.