There is no comparison at all, the wiiU is similar to the wii which is also similar to the gamecube architecture wise. The ps4 and ps5 are totally different from the ps3. Games would have to be changed, microsoft is doing it for the xbox one because the 360 had a similar architecture, so the work is a lot easier.

Even a powerful PC can't run ps3 games 100% bug free.

Also sony has no incentive to do it, they know ps5 will sell really well without having to work on backwards compatibility. Microsoft on the other hand, they need it to attract customers, in fact one of the biggest reasons I prefer my xbox X over my ps4 pro.

Regarding PS3, most of its games are already available on PS4, except a few gems. They could just work on those and make them available on PS4/PS5 or PS Now.

PS2 and PS1 could come back as re-released classics. They could select the most acclaimed games. Having full BC with PS2 and PS1 would benefit some players, sure, but most current gen players wouldn't give a damm. I think it'd be a waste of work. Unless we're considering plain software emulation which we know that will never happen due to many technical reasons that many don't fully understand.

The problem with PS3 games is the complexity of the CPU and thus the complexity of emulating instructions. I have no doubts the PS5 could run PS3 games judging by it's specs and what kind of PC you need to run RPCS3. The problem would be getting enough games to run perfectly. They could just as well use RPCS3, implement a compatibility list and try to get as many popular titles compatible.

Will they do it? Probably not. But it would net Sony a lot of goodwill.

Sony is not going to add PS2 or PS3 BC at all. They want to sell you the PS2 and PS3 mini classic consoles with 20 preinstalled games on it for $149.99.

That would be great considering people like myself who bought a ton of PS1 and 2 classics off the PS3 PSN digital store which are now locked onto that specific platform. It only does everything is no longer a care for Sony apparently. 

You don't need to emulate the instructions. - There are other ways to approach that horse.
Binary Translation for example, you intercept an instruction and "translate" it into a similar instruction that can be executed by the new host machine.

Emulation is intrinsically wasteful on resources... There is a reason why the Xbox One with it's woeful Jaguar cores is able to emulate the Xbox 360 with an uptick in framerates and frame times.

The PC isn't representative of what Sony can achieve, PC is taking the approach of reverse engineering to achieve backwards compatibility. (I.E. Lots of trial and error.)
Sony has the information to build an emulator from the ground up without needing to do any of that, so they can make an emulator with significantly less overhead as an instruction isn't being split up and interpreted as multiple other instructions which results in a corresponding hit to performance.

The complexity of the Cell is a poor excuse, especially as the PS3 emulator is better than the Xbox 360 emulator.

The Cell is actually a little easier to emulate, because it uses so many "simple" low-performing cores which can translate really well to many-core powerful PC's, it was hard for developers to build games for, but it's actually a benefit to emulation.

The power is in the duct tape you need high bandwidth tape.

PS5 have BC with PS4 baked in the chip itself. And since they haven't mentioned anything about PS1 to 3 I wouldn't count they having even if it was possible.

More than that, they like to sell the same game just ported for 5-10.

BC on Xbox has also been monetized though. There are OG Xbox and 360 games on Game Pass. You can buy most titles on BC as well.

PS Now could actually benefit from PS1, PS2 and PS3 emulators on PS5. It would mean you can play games on your hardware, which is better than streaming.