Google suggests stored lead acid batteries lose about 5% of their charge per month. More in warmer climates. If you have clocks and automotive electronics running off the battery as it's stored, the loss will be greater so disconnecting them is a good idea.
While I cannot comment on the newer generation of AGM batteries, which I am quite pleased with, flooded lead acid batteries don't hold up well to over-charging. Some battery tenders, operating 24/7, keeping the battery at 100%, can do more harm than good. I'm pretty sure at least once, I experienced a battery tender ruining a battery that I had replaced the previous spring. A Mopar restoration one I went to great effort and expense to acquire. When you check on a car and battery in a cool storage area, and find the battery to be warmer than ambient temperature, this may be a red flag.
Back in the 1990s, I worked as a software engineer for a credit card company, and they had a backup battery supply for their main frame computer. It was essentially a whole room filled with 100s of lead acid batteries connected in series to a transformer. These looked like typical 6-cell lead acid motorcycle batteries. Every month, it was my job to go inspect the fluid levels in each battery, and top them back up to full when below the fill mark.
They found when on a battery tender circuit, to keep them constantly fully charged 24/7, the batteries lasted about 2-3 years. But when they switched the charging cycle to weekly, kicking on one-day a week, and charging until capacity was reached, the batteries starting lasting considerably longer. They were trending to last 8 years when I left.
If I were to require a battery tender again, regardless of it being made in china, or it's cost or quality, I would be inclined to put it on a timer, only to kick on a short time each day. It probably takes 1-2 hours of charging per month, to bring the battery back to full. That's about 5 minutes per day. But I think ideally, kicking on once a month for that 1-2 hours would be even better.
When I store a car I don't even plan to drive in the spring, I've had good results pulling the battery.