Many reservoirs or even CPU blocks made of plexi, start to leak in some time (usually after about a year) as under pressure are appearing little cracks (often a bit too tightly screwed fittings). I had that with most blocks where tops were made of plexi and the fitting holes were not protected (metal rings or something similar). The exception are graphics card blocks where are almost always tops/replacable terminals. I haven't seen them cracking after a long time of work.
Some blocks are not designed for higher temps. Like previous generation of Aphacool CPU blocks was leaking when CPU was running at high temps and the liquid temp was higher than 60°C for some longer. This is also the reason why some manufacturers clearly say in specs that the liquid temp has to be below 60°C. I don't know exactly what was the problem but I saw two different Alphacool blocks leaking after longer usage in SFF where temps were higher than supposed to be (like 60°C+ liquid temp).
I also had leaking Phanteks block. The top was fine when I installed it. After ~3 months were a lot of little cracks and dried liquid that was dropping on the graphics card. Simply material quality was not the best. The same story was with cheap Chinese reservoirs some years ago.
If I buy any blocks nowadays, then I try to search for good looking full metal or acetal series. I'm not a fan of EK as their products are expensive and not always looking as good as on pictures, but they have the widest offer of acetal CPU and GPU blocks. Most other brands have almost only transparent GPU blocks. It's hard to find any reservoirs other than transparent regardless of brand. Most brands have metal/acetal CPU block series, but not reservoirs. At least these well-known brands are making improvements each generation, so plexi is much stronger than it used to be a couple of years ago.