If not done properly it's possible to degrade handling. Depends on the bike.
But could be OK too if geometry is altered to suit. Not sure about a fat wide front tire riding on road ...
If your bike normally has a 21" front ... then going to a WIDE 120-18" will certainly change things. Not only do rake and trail change ... but all the subtle geometry your whole bike was designed around. But if you run the same size rim up front as on the rear ... then you've got a very wide tire up front ... which might suck or be weird, dunno?
A 120 rear is pretty narrow for a rear tire ... so you might get away with it. Lots of guys on the DR650 forums have gone from the stock 21"/17" combo to either a 17"/17" Super Moto set up ... or a front 110-19" and maintain stock 130-17" rear. Reports are mixed ... but the 19" front has promise ... at least on the DR650 and if set up right seems to work fine. Most changed suspension completely, adjusted ride height or made other changes to get the bike to handle.
Overall I'm not sure it's worth it.
The front 21" tire on my DR650 lasts 2 or 3 times longer than the rear. So what's the problem? I don't carry a spare front anywhere ... if I start with a new one ... I can go nearly 10,000 miles before it needs replacing.
Rear tire is another story. 3K to 5000 miles depending on tire and usage. I carry a rear. But a front can nearly always be found somewhere within a 10,000 mile range on the road.
Also, fronts tend to be cheap and more plentiful. Much more than a 120-18".
In Latin America 17" rear tires are now fairly common, not hard to get.
Thank all the bikes that use 17" rear tires: KLR, DR650, F650, BMW twins,
Vstroms and lots and lots more. Many shops now stock them. But talk to any KTM rider looking for a 130 or 14-18 ... or any 18". Not common at all.