Meat always come out rubbery and dry!

I wouldn't buy a leg of lamb for roasting fom Aldi full stop. There meat is cheap for a reason. Cheaper cuts, fine. Braising, casseroling and stewing were developed to be used for poorer quality meat and lower grade cuts.

If I want a joint - and I rarely do, I would go to a good butcher or farmshop.

I cooked a gammon from one of our local farm shops in a slow cooker at Christmas. I put it on the lowest heat and let is cook for 24 hours. I cooked it in red wine and it fell apart like pulled pork and was so delicious we nearly didn't bother to cook the turkey.

I didn't know lamb could go dry! It was either rubbish quality or over-cooked. I'm tending towards the former.

I also didn't know one could do a roast in a slow cooker. Isn't slow cooking more for casserole type meals?

I would suggest it’s the quality. Perhaps trying an organic butcher might let you compare the difference.
We use a couple of organic butchers ( supporting local community) and find you actually need less quantity because the meat is such good quality. It’s nit pumped full of water or preservatives and therefore doesn’t shrink in the cooking.

I buy leg of lamb and always roast it in a roasting bag. No need to add any oil as lamb is a fatty meat.
Roast it for approx 90 mins if boneless like mine are (from Sainsbury’s, British) and take it from the bag and let it rest with a clean tea towel covering it for 20 mins before slicing.
Always cooked through, (don’t want it pink), soft and delicious.

When I bought my kitchen a lady came and showed me some tricks on using the appliances and cleaning. I didn't think it was necessary, but once she was here I did learn a lot. One thing was that if you kept the oven at 165° or less, the meat would not spit and the oben would stay cleaner. Also, cooking the meat slower would make it more tender.

It certainly doesn't seem to matter not having it at 180°. I haven't managed to keep it clean that way though because there are some things I do cook at the higher temperature.

I'm a bit dubious about the quality of the meat and poultry they're selling at the moment. so many delays in production and supply.
I wouldn't cook lamb in a slow cooker either. Different makes have different timings. Better in a traditional oven.
I've recently started to cook chicken (joints) on a lower heat. After smearing it with a sort of marinade and leaving it to absorb overnight. much softer and juicier. Maybe that would work with lamb too?

Heat oven to 200 degrees C (180 fan). Ensure 2 kg leg of lamb is at room temp. Rub leg with oil, salt, rosemary, garlic etc etc to taste. Place uncovered on a rack in a baking tray, and cook for 75 minutes for pink, 90 minutes for well done. Remove from the oven, and rest covered in tinfoil for at least 20 minutes. Works every time. Even better - use a meat thermometer to confirm the ideal internal temperature.

I just bung roast lamb into a preheated oven in a tin for just under the time on the label. Take it out and let it rest. Lamb is best if pink in the middle.
Aldi and Lidl have the best meat imho.

I’ve never been happy with any joint cooked in the slow cooker.
I put a joint into the oven at 180. Well wrapped in tinfoil with a little water. Cook it for much longer than any instructions. If label advised 90 minutes I’d do it for two hours. Take the top foil off half way and keep an eye on it. If necessary put foil back on. After the two hours I take it out and wrap in the foil. Cover with two thick towels and let it sit for 30 minutes. It will still be hot. I think resting for only 10 minutes isn’t long enough.
Mine never fails. Always tender and moist. And slices beautifully.

if it is a leg of lamb i'd just cook it in the oven but never for as long as they advise on line. In a slow cooker presumably you just cook it slow until it does go 'soft'? like a shank you mean>

If it isn't a good cut, could you try braising it (covering it in water/stock) instead? It would be tender.