Nice thing about nice ones like the Morningstars is that they sense load to balance charging against usage. They also can do equalization which is nice to keep batteries conditioned. Sometimes you don't get a good charge because your load take priority over charging while other times you have excess power during low usage that could be used to condition the battery. They can also remote temperature sense the panels which is one variable used in the MPPT algorithm to determine peak efficiency.
The risk for simple chargers is mainly if they don't do a good job and over/under charge your batteries. Going over voltage is bad, leads to sulfation and ruins the battery.
It's more likely that they starve them perpetually of current. If you remember time and current are the two variables in the battery charging equation. There are specific voltages and currents required to fully satisfy a battery during all phases of charging.
It's mainly during bulk charging that you will need to give the battery the current it wants and this is measured in percentage of capacity (C). So a C of 100A-hr might require constant current for 8 or 10 hours at 0.1*C at 2.35V per cell or 14.1V for a 12V type. If you dip below this you will starve the battery, resulting in insufficient chemistry reversal but exceed this current and you overheat the battery. Heat is a big enemy of batteries especially during charging. Usually once a battery hits stage 2 or 3, what is usually called maintenance or floating, the internal resistance begins to climb and they regulate their current themselves. It's when you are starting to charge a deeply discharged one that you have to be careful to give them neither too much or too little current.
None of this is to say that you /must/ spend a lot to get a high quality charge controller, but often you do get what you pay for.
'91 Pickup - Imelda
'08 Tacoma TRD
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche