Addendum to that (I wrote that right after I woke up...felt kinda snarky)
IME, you are probably talking to someone who reads the manufacturer's recommendations and takes them at face value. This happens a lot.
Here's a little story that will shed some light on why I say this.
I spent a lot of time on Google trying to find the best method for tapping large holes in acrylic (for my algae scrubbers). What I found out is that google doesn't help you find the answer to everything...far from it, in this case. The closest I got was an article from 15-20 years ago that was written by a guy who worked at Cyro (they make Acrylite) about hole tapping, but it was mainly w/r to small holes and not ones that were all the way through. Anyways, on a whim I called Cyro and asked for him, and lo and behold, he was still working there.
He was a very nice and helpful gentleman. He had been working for Cyro for something like 30 years, probably longer than most of his co-workers. He didn't really have an answer for my question on hole tapping (hmmmph) but I took the opportunity to poke his brain on bond strength questions. To make a long story short, he told me that 2-part adhesive was what he always recommended, and "never solvent welding for aquariums" and that is pretty much a quote.
I proceeded to tell him that many of the top manufacturers use solvent welding for tank up to 2" thick. His response was basically towing the line on bond strength based on the solvent properties. So I dug further. I asked him how they determined that a solvent weld was inferior.
His answer was quite telling. I can't recall if he utilized the pins method or not, but at this point I'll assume that he did. What they did was form a 2-part joint, and then a solvent joint, and then tested them to see which broke easier. The solvent weld broke easier. When I asked him how long they let the joint cure, he said it was something on the order of a few hours.
Problem!
Solvent joints do not fully cure in just a few hours. Set a pinned joint and then try to break it in 2-4 hours, and the joint will break at the weld. Let it sit for 24 hours or more, and the material will break before the weld does. When I told him this, he didn't really have an insight, because...they hadn't tried that.
The joint curing time is a critical factor in joint strength. Not touching the joint - not moving the piece in progress AT ALL for 20 hours, is critical.
Now, back to the weld strength. As far as I know, the acrylic aquarium fabrication industry is the only "division" of the acrylic fabrication industry that promotes and utilizes the "pins" method. Ask any local fab shop what this method is that you'll get the deer-in-the-headlights look. Explain it to them, and it's like a light bulb lit up in their brain.
When it comes to solvent welding bond strength, the entire acrylic fabrication industry is highly focused on bond strength charts, but those chart for solvent welding almost always (IMO) are written based on capillary welds. At best, they are based on capillary welds where the joint is kept separate by leaning one piece a bit, running the solvent, and then immediately putting the 2 pieces together (which, note, you cannot do for the top/bottom panel). Minimal soak time. This does NOT create a strong joint, so then the immediate conclusion that the industry reaches (based on the misleading joint strength data they are handed by the manufacturers of material and solvents) is that 2-part is much stronger. While that may be the case, it's completely unnecessary to make that leap because when solvent + pins is used, it's really not THAT much stronger.
Capillary welds snap clean. Pinned joint welds snap at the material. You can tell that a solvent joint is strong immediately after it is cured. You can only tell that a 2-part joint is strong when you fill the tank and it doesn't explode. Nearly every tank you've seen that has had a catastrophic seam failure that wasn't a material defect was a 2-part joint (big cylinder tanks, etc). When they go, they pop like a capillary weld joint.
Now it just occurred to me that I didn't really answer your question.
They both recommend using a UV cured adhesive for building a tank
No one (that I know of) in the tank building industry uses UV resistant material. So why would your solvent or adhesive need to be UV resistant? The answer is, it flat out doesn't.
