Is it hard to do 2 sided SMD PCB in oven?
No, it’s generally not a problem. You can solder the light parts first and then flip it and solder the rest. The surface tension of the solder will hold small parts onto the board.
In the unusual case where you might have a heavy part or two on the bottom on the second cycle you can add adhesive or solder it later by hand.
'Heavy' relates to the relationship between pad area and weight, with a rule of thumb limit of about 30g/in^2 of pad area. So, for example, a 1206 resistor with 0.005in^2 of pad area has a limit of about 0.15g. The actual mass is 10g/1000 or 0.01g each.
See this accepted answer for references.