Space Marines are strong, but people use them wrong. A good space Marines template is 10 HArm 10 Marines, its essentially a cheaper form of the 10 Mechanized 10 HArm divisions meant to stop tanks. I don't ban the Marine form just like I don't ban the Mechanized form.
The point of Space Marines is a general purpose division template that can replace your line infantry...
Your not going to afford replacing your line infantry with 10HARM+10Marine templates!
While it might have some use as a very expensive specialist unit to force rivercrossings and amphibious landings, it would be a horrible thing to use instead of line infantry since you could afford just a few of them leaving big holes in your lines.
It's also a big waste for everything that don't need the marine terrain bonuses ( like fighting in plains or other terrains without rivers ) since you combine the worst and so many marines dillute the valuable hardness & armor bonuses of HARM, as well as slow them down to minimum base speed of 4kph which combined with HARM terrain penalties means they are crawling.
It's also lacking in the soft attack department compared to it's cost.