What you give up, honestly
A multipurpose bar is slightly less stiff than a power bar, so at very heavy squat weights it moves more than a 29mm shaft would. It usually spins slower than a dedicated weightlifting bar, so a snatch feels less fluid. If you push hard in one direction, you will eventually want the specialist bar.
But "eventually" is doing a lot of work in that sentence. Most people who squat, bench, deadlift, press and throw in the occasional clean never reach the point where the compromise costs them anything.
Dual knurl marks
Powerlifting marks the shaft 810mm apart; weightlifting marks it 910mm apart. A dual-marked bar carries both sets of rings, so you can find a competition bench grip and an Olympic snatch grip on the same bar.
It does mean four rings on the shaft rather than two, which takes a session or two to stop noticing. Strength Shop’s Riot Hybrid and Mirafit’s M3 both do this.
28mm against 28.5mm
Half a millimetre is a smaller difference than the specification sheets make it look, and both are perfectly sensible. 28mm matches the men’s weightlifting standard, so the bar feels familiar if you also use a bearing bar. 28.5mm is a touch stiffer and is what Rogue uses on the Ohio Bar and the Echo Bar.
If you have no strong view, ignore it and choose on steel grade, sleeve construction and finish instead.
The finish question, which matters more at home
In a commercial gym, bars get wiped down and the room is heated. In a British garage in February, a bare steel or black oxide bar will develop surface rust if you leave it alone. Chrome, stainless and Cerakote all resist it; bare steel and oxide grip better and need looking after.
This is a real decision, not a cosmetic one. Several bars here come in multiple finishes at different prices for exactly this reason.