So I'm running a campaign that has about early 1700s level technology - gunpowder, steam engines rarely.

I know that DnD has official (albeit optional) rules for firearms now, but what I've always done is reflavor crossbows as firearms. Hand crossbows are flintlock pistols, heavy crossbows are muskets, etc. I've usually left out light crossbows because I agree with the PHB that firearms should be martial weapons.

Is there any problem with doing that? My reasoning is that it allows people who want to use firearms to interact with more feats and magic items and stuff - crossbow master, etc. It also keeps the damage of them more balanced in my opinion, since I think weapon attacks should be generally weaker than cantrips since you can make multiple weapon attacks with a bunch of classes.

There is no problem at all. 
Personally, it’s what I prefer to do, as it keeps everything within the standard balance of the game. Re-skinning something is a great choice.

To make it simple, basically ban steam engines except on a case by case situation.  

As for firearms, you may even want to adjust them a bit more.  Accurate to 30 feet,  beyond they get a -1 to hit, maybe add in a -2 at longer distances.   This is in addition and not ignoring current DnD long range and cover shooting rules.  You still use those. You can maybe balance by increasing the damage by +X.

As items were muzzle loaded, the ball was never tight against the barrel, so it "rocked/bounced" as it traveled.  Thus if it was bounced off the side, just be edge it would not be a straight trajectory as it leaves, giving you  a large miss.  It is why the soldiers were shoulder to shoulder when they fired.  You aim at person A, but hit person Q.

Attempting to model the actual accuracy of 17th Century firearms just means that no player will bother with them unless you ban bows.

Why not keep bows in your game?

If a player wants to use a crossbow, I let them, as one of my players does, but for those who'd rather flavor it as a firearm I allow it as well. I prefer firearms to crossbows for the time period my game is set in, just as a flavor thing

The more games I play in and I learn about dnd mechanics the more it makes me realize that reflavoring is one of the biggest keys to RP. There is no need for tons of new mechanics like I see some people request. A little flavor and RP can solve so much. I love the idea of just flavoring crossbows as guns. No need to fiddle with any of the other mechanics because they already work. 

This is fine. But there is just one little detail that I would add. in the early 1700s, firearms were still in the experimental/developmental stage, while crossbows were a proven technology with 2,000 years of effectiveness under their belt. This meant that both could do damage at range, but the crossbow was more reliable and easier to maintain, while the firearms were a lot more... finicky.  So I would simply reskin the crossbow as a firearm BUT if the wielder rolls a Natural 1 on their attack roll, the firearm malfunctions. Either it fails to fire (either due to a jam or a pop & no kick), or it fires but in doing so does damage to the weapon. Now the wielder has to spend an action to clear the weapon before it can be fired again. And to do so requires proficiency in tinker's tools.  I'm pretty sure the (optional) rules for firearms have something like that already built it. You could follow those rules or just simplify it by saying basically a Nat 1 means you lose a turn fixing the weapon.  And in exchange for this drawback (even though it's just a 5% chance) you could grant them a longer range, or whatever. 

Just my 2 c.p.

In the 18th Century, firearms had been in use for 800 years.

I never said firearms were new. I said they were still in the developmental phase. I have fired a Brown Bess. They had a slow, multi-step reloading method, were not terribly accurate, and they were extremely finicky. Crossbow wielders don't have to worry about keeping their powder dry. I'm just saying, crossbows had already been in use for millennia. There's something to be said for that.