Everybody know that Hybrid bike can be used for mountain cycling. But I want to know what are the disadvantages if we do so? And what are disadvantages of using mountain bikes in cities.
Since a hybrid is a combination of two or more types of bike characteristics, there are many possibilities, and you should consider what aspects of the particular bike where meant for the mountain or for the road and see if they meet you needs.
You can use almost any tire, provided your frame has the clearance needed. Wider tires can be safely used at lower speeds and also provide a contact patch with a larger area, both give better rolling characteristics for off-road riding (more traction, shock absorbtion and lower rolling resistance on rough terrain). The rougher the terrain you want to ride, the fatter the tire you want.
Some Hybrid frames won't accept tires wide enough for serious MTB riding. On the other hand, a wide frame and fork can accept the thinnest tire that fits your rims.
Generally, MTB requires low gear ratios to provide mechanical advantage for ascents on rough terrain or for extreme climbs. On the other hand, Road riding requires high gear ratios to provide high travel speed while keeping the cadence (pedal rotation speed) relatively low. Commuting requires a mid range gearing, which you may modify towards any of the two extremes depending on how hilly is your city, your style and physical condition.
A road drivetrain may not have the low ratios needed for off road riding. Generally speaking, the same slope is rideable faster on pavement, thus you can use higher gears. An offroad slope has many smaller obstacles like roots, stones and terrain irregularities that do not allow the use of high ratios.
A secondary disadvantage is that larger chainrings get closer to the ground and so, are more likely to hit obstacles like logs or big stones.
Mountain drivetrain is normally good enough for smooth commuting. For a flat city where you don´t need higher speeds it means that only the mid-range gears would be used. The low gears may come in handy if you commute on a hilly city with steep climbs.
However, for high speed road riding, you may run out of gears.
Traditionally a road frame has an horizontal top tube, this reduces standover clearance. (distance between the top tube and the crotch of the rider while he or she is standing with both feet on the ground). Some hybrid bikes are designed like this. On mountain riding you usually need greater clearance, making this a disadvantage of certain Hybrid frames for MTB use.
The other disadvantage may be that the hybrid frame may not be properly reinforced to withstand the high demands of serious MTB riding or may not accept upgrading certain components, like a fork with suspension (or greater travel).
On the other hand, MTB frames tend to have sloping top tubes to provide better stand over clearance. While this is not properly a disadvantage for road riding, or commuting, the lower top-tube and seat tube junction also lowers the possible mounting points for rear racks, (if any) which in turn may reduce the possibility of fitting one. This is a disadvantage only if you need a rear rack with medium to high load capacity. (Because there are seat post mounted rear racks with limited load capacity).
Suspension forks and frames.
Some hybrids have mountain geometry but rigid forks. These can be used on light offroad use, specially for prepared trails that have been smoothed out. And for city riding if the city has good infrastructure (no potholes at least).
A suspension fork with around 100mm travel is good for any riding you can do on a hard tail frame (non suspension frame). It's excellent for trail riding and for cities where potholes are common or if your riding involves a lot of getting on and off curbs.
If you don't have a fork with suspension blocking, you should at least have one with preload regulation and rebound control. Higher preload and slower rebound setting than used for mountain riding helps to reduce pedal bob, thus improving efficiency for city riding. The same goes for using suspension frames for city riding.
A suspension frame is very unlikely to have mounting points for rear racks and complete mud guards.
My personal experience
I have successfully commuted on a very hilly city and made two 500+ kilometers rides (with around 100 km stages) on a hardtail mountain bike with slick tires, an air suspension fork, 3x9 mountain drivetrain, straight handlebars and a seatpost mounted rear rack.
The low gears (22/34) have been a blessing on steep roads or when I get really tired. On the high end, my 42/11 top ratio has been enough for the speeds that my training level allows me. The straight handlebar provide only one hand position, which causes hand and wrist strain on long rides.
The mountain fram geometry allows for a more upright position. This proved good for the back o my neck, since being 4+ hours on a bike at a time caused pain on my peers that where riding road bikes.
The thin, slick tires are so great that I can almost roll as easy as my road counterparts, but they are not as skinny as theirs, so I get slightly more comfort on rippled or highly cracked asphalt and more confidence against potholes.
The air fork allows me to easily tune it, using higher pressure to reduce pedal bob, since it doesn't have locking. The rear rack is very useful for light loads, but it vibrates a lot more than a frame mounted rack.
Another advantage is that my frame has v-brakes that perform a little bit better than regular road brakes.
My bike fits my needs closely because I didn't bought it as is, I have changed many components over the years. The only change I want to make now is to incorporate a road or a touring handlebar. (but I don't want to change my brakes or shifters...).
I'm currently experimenting with a rigid fork and bar tape on a riser (MTB) handlebar.
Bottom line is that bikes have so many components that the possible combinations are almost endless. There is no possible way of making a bike that will excel in all types of riding, let alone one bike that fits everybody's needs.
This means that any bike will have disadvantages for certain type of riding, so it is up to you to define your needs and pick the bike that most closely meets them, experiment, research and decide whether changing some component is necessary and possible (considering budget and compatibility) to make your particular riding better.
Frames very similar to current hybrids have often been sold as mountain bikes for fairly benign riding. With a suspension fork you essentially have a hardtail. Mountain tyres tend to have much more tread which is a bit of a chore on tarmac. For many forest trail rides you could just get on a stock hybrid and ride in the dry at least. Anything much more serious and you'd need to change tyres and possibly the fork. For the really rough stuff you'd want full suspension so the downside is you can't ride that on a hybrid.
Mtbs in the city - it depends how far and fast you're riding and how much suspension you've got (especially if it can't be locked out). You may also lack pannier mounting points for commuting. But people do very happily commute on full suspension mtbs, especially in the winter if they've got a road bike they want to keep clean (or maybe they prefer the handling of the mtb in the wet/snow).
The key difference between a hybrid and mountain bike is the geometry and the position of the rider.
Hybrid bikes tend to have a steeper headtube (headangle) which gives the rider a more upright position. The disadvantages for climbing will be the riders weight would be too far back and the bike would feel as if the front wheel is lifting. Descending the rider would feel like they were hanging over the handle bars.
The disadvantages of a mountain bike in the city is that it has a more aggressive riding position and can be more tiring than an upright position. If suspension can't be fixed or locked-out there is also efficiency loss with pedal bob (bike moving up and down).
Tyres, drive train and suspension are interchangeable but not the frame.