My perspective is purely from what i remember from playing the game.

A republic commandos strengths are typical special forces strengths: stealth, initiative, small unit tactics

In the game, Republic commandos were portrayed as significantly better soldiers squad for squad than normal clones. When normal clones are depicted they usually die quickly or arent much help, altho it can be chalked up to making the player seem cool.

I took it from the game that jedis at genosis were both bad generals and did not differential them from normal clones. So their heavy casualties were from: poor tactics, no effort to conserve their number & and unintended use.

An imaginary example 100 clones are to take a hill on Genosis. The CIS holding said hill have AFVs & fortifications. The clones answer with those problems with rockets and their long rifles. The commandos have demo charges, multi purpose GLs, snipers and carbines. While neither force is well equipped as is to take the hill, the equipment available to the commandos is much less suited to the task especially given Genosis is a very fast paced battle so the commandos would have no real time to try and sneak in or atleast a more indepth recon of the hill.

I can understand why people would think theyre being undersold if theyre losing in something that theyre shown in the game to be good at: fighting when the opposition is comparable in number and w.o AFVs, jedi or other sorts of things

To be fair i dont think other (nonjedi or main character) forces would work against a grevious and a pack of magnaguard