No problem and it really depends. If you don't like fiddling with things than no, something like an ego-T isn't for you. Since there will be times where you will have to fiddle with it (for ex. cleaning, dry hits, leakage). Leakage seems like a common occurrence since the people that don't like the tank system are more vocal about. So, if you look at it that way then yes, it's pretty common. There are preventative measures that can be taken to reduce the risk of leaking. It just depends if you want to do it or not. Such as buying soft plastic caps for your tanks or adjusting the wick to reduce the amount of liquid that goes into your atomizer.
You also have to use a specific mix of e-liquid with it. Such as a 80PG/20VG or 70/30 since it wicks better in a tank such as an EGO-T. Also, the e-liquid has to be fairly clear since the darker liquids can gunk up the coil in the atomizer. You will then have to pop off the piercing plate to clean it. Which takes a little bit of work and there are a good number of people that don't want to go through that hassle.
Carto tanks will stick out like a sore thumb and so, that won't be good for a stealth vapor. It is however, more convenient. Which is why people like it since all you do is pre-fill the cartomizer, fill the tank, and your good to go for the day. The only other alternative out there other than what I've mentioned are bottom feeders. Which is what I use. It is box shaped and allows the user to carry either 3-6ml of e-liquid anywhere they want. Also, it will give them better battery life as well. If you want something more stealthy then a REO mini is a great choice but, the batteries will only last 4 hrs or so. Meaning you will have to carry around extras. Not sure if you mind the form factor or not but, it's another choice out there for you.