The reason Sikaflex is not recommended is because its designed as a bonding sealant. But for the purpose of only sealing, I think it works great. You could also rough the surface to maximise the mechanical bonding If you are worried about it.
The only places I don’t use it are where I think I will have to disassemble in the future. Sometimes it works a bit too well.
Sika 291i just to name it has a well deserved reputation in marine industry. Having stacks of that, I used it extensively on my board but on PMMA or perspex, plexiglass and PP it failed. Chemically attack the surface and make it soft. Now I tried a different sealant/glue, silicone free, not famous brand but good for pmma also. Let see. I glued PLA halves of my remote with that, before to lay epoxy on top… It should work.
Yea, Sikaflex recommend 295UV for PMMA and PC. Also some plastics require primers to work properly and who can be bothered with that. I suspect release agent chemicals added to the batch are part of the problem.
Last month I was in a rush and picked up a tube of Sikaflex 552AT by mistake.
It is an automotive sealant and I have had good results with this also. It is solvent and silicone free.