Yes. several problems but chief one is depth of glass rebates in frame which used to be less than now. Easier for fixed (non-opening) panes which effectively have deeper glass rebates, but more difficult for opening sashes. Mostly I made new opening sashes.
Problem you will encounter in (no particular order).
Single glazed glass held in place by hardened putty makes the whole frame rigid. That helps a softwood frame (which of course is normally weaker than a hardwood one) stay square. But DG panels should not be fixed in rigidly otherwise leaks will occur and contamination of the gap will occur (condensation etc). That means you need to take care that softwood sash frames are well protected from damp (ie well painted and primed) and corner joints are rigid.
There is a conflict between optimum spacing for heat insulation and sound insulation. Regret I can't recall the exact figures, but IIRC the nos are around 15mm for heat and 20mm for sound.. Wider gaps encourage convection currents in the trapped air which aid heat dissipation. In any case few single glazed sashes will be thick enough to allow more than 4-6-4 DG (ie 4mm glass+6mm gap+4mm glass). IIRC the norm is now 4-14-4 and new timber frames have been thickened to allow for that. I changed SG to 4-6-4 DG in the smallest room (typical 600 wide window with horizontal opening storm vent at top) a while ago in winter and the increase in comfort level was noticeable immediately and substantial. The room was no longer draughty and the radiator was turned down a notch. Possibly there wouldn't have been much further improvement with 4-14-4DG?
Some posters here question the economic justification of DG. That's is very possibly correct, prospective life of the units possibly being short maybe only 10 to 30 years? And I have little doubt that energy used in production is a goodly proportion of potential energy saved. However IMHO the increase in comfort levels from draughts caused by SG more than makes up. Further DG is paid for at today's prices and all I can see is energy prices escalating in the years ahead at a far greater pace than inflation, so future heating bills have been capped to some extent.
DG units are easy to obtain, just ring round glaziers in the yellow pages. Get several quotes: I found prices can differ widely. You will find some will take your order but contract out the work to a different firm with the fabrication machinery. Avoid them - ask if the units are made on their premises. Also because of FENSA and the building regs monopoly they have, ask for DG units without date stamping (usually fabricators print the date made in dot matrix style on the separating plastic inside the gap. That will hinder future home inspectors demanding a FENSA certificate.
Fixing the units: Sideways (in the plane of the glass) there must be a
3mm gap between unit and the timber frame. My glaziier gave me a bundle of hard black wedges about 3mm x 8 x 20 for this (use c. 400mm apart). The unit must NOT be set in hard putty. SFAIUI the corrrect stuff for a timber frame is a non-setting butyl material. The only make I know of is Flexistrip made by Hodgson Sealants, Beverly (tel no in 2001 was 01482-868321) [I've just checked they now have a web site- you could try asking your DG panel fabricator for a fixing & supplier recommendation. Flexistrip comes in reels of double sided "tape" style, about 1.5mm thick and 12mm wide and is incredibly sticky (IIRC different thicknesses and widths are available). Just unroll it into the rebate then carefully drop the DG unit in. Once the DG unit and flexistrip make contact you will not be able to separate them cleanly. If the fit is not accurate & you have to redo the job then the Flexistrip is wasted. You can't fudge an adjustment or retry as mucked up flexistrip seal shows through at DG edges & looks awful. You must take out the panel and reapply the Flexistrip.
Finish by sealing the DG panel in with glazing bead NOT putty. Finally plug any small gaps with a mastik sealant such as a bathroom sealant then prime and paint thoroughly.
If fitting an obscured unit in a bathroom etc then fit with the normal plain glass side of the DG outwards. That makes for more reflections, improving the obscuration effect.
HTH
Good luck