The biggest issues with all wet cell batteries constructed of lead plates sunk in sulfuric acid can be summed to one word: Sulfation.
Sulfation is a break down in the acid solution where sulfur solids separate from liquids and collect on lead. When voltage is allowed to drop - either by self discharge or demand from an external device - below a minimum level (about 10.5VDC), sulfur collects on lead. If voltage is allowed to drop several times below 10.5 to 11Volts, sulfur collection will chemically isolate the lead from the acid solution. When that happens, corrective action is "boiling" - which is supplying 14.5 VDC from a properly regulated external source. The presence of 14.5 V initiates a chemical boil (AKA: equalization), dislodging sulfur from lead, causing the acid to achieve correct specific gravity, restoring the battery to rated capacity. Boiling causes significant outgassing from cell chambers and must be vented into open air because the gas is explosive. And were there is gas, there is the potential for pressure, so a boiling battery needs to have old school caps.
The problem with so called "maintenance free" batteries is they can't be boiled. At least, not safely. These 40lb toxic waste "disposable" nightmares were concocted for the sole reason of removing a simple maintenance requirement from vehicle owners.
Any wet cell battery whether serviceable or "sealed" brings with it venting of caustic gases that cause lots of service issues, including terminal corrosion and other wiring issues we see under the hood of poorly maintained vehicle electrical systems.
AGM and gel batteries have distinct functional and service advantages. They *can* sulfate but don't at anywhere near the rate of liquid cells. And they don't outgas either, saving cables and greatly reducing the chance of cable/wiring failures during the life of the battery.
Glass mat and gel batts cost about twice as much as a premium wet batts but in my opinion, they are worth every single penny. Lower or even NO maintenance, little or no terminal corrosion or ancillary damage from outgassing, and FAR superior resilience to continuous dips below 11VDC. It's a no brainer in my book.