Was not that long ago on tool talk you mentioned this, darned simplest things always cause the most trouble.
I can recommend Lincoln, the pricey battery powered model, was very reliable, I used it to grease a '08 JD 637 disc, lots of fittings, all the tractors, 3150, 4440, 7420 JD's, a mack tandem sileage body truck, and not to forget the 6620 combine, + 315 NH baler. Not to forget the ole 620 JD, and all the other equipment, NH rake, the tedder, and so on, only trouble was if you forgot to recharge a battery, we had 2, this was a good tool, thats a lot of fittings, forgot the 550 JD dozer left on the farmers land, contractor was dumping clean fill, we had use of it and I always take good care of equipment in appreciation. Same contractor would stop by the shop with an excavator on the trailer, I would grease it, very nice people, always helped us if we needed a dozer, or excavator, I felt like a good natured old school service attendant when he stopped in. That Lincoln sure did the job, saved me time, and made short work of the job, often times the farmers son would be coming in after the day job, and I'd have whatever was going to the field, ready, just get in and go.
Alemite was always a good quality brand, look at the history of the company, long time in that business. Napa used to sell a good quality gun, I have had 2 one was '91 or so, another in '05, no issues with either of those. This is a tool I use very often, so I know the frustration. I had found the darned ends or tips were a pain in recent years, so tight they pull barbed fittings out, (I am not a big fan of barbed fittings) or have other problems. Depending on what I am working on, I like a rigid hose or flexible, sometimes one gun with each, especially when you have to use both hands to hold it so it does not leak. Thats the bonus with the battery powered model by Lincoln, if you need a hand to hold the tip, say for a funny angle, where it will just leak if you don't hold it just right, you have a free hand to do both, hold the end, and squeeze the trigger. A cartridge hand pump gun can be a pain, flexible hose, or rigid, when you need an extra hand and you end up having to line up the pump handle to something you can push it against, to pump grease. I've had some of the pump type cartridge guns get air bound or something where fiddling with the release, pumping etc. these can be really frustrating. Right now I am using one from our service truck that was from the late 60's, ford tractor dealership, it always works fine. Keepsake cause it has our sticker on it, but the darned thing works just fine.