Vegetarian slow cooker fan here.
They're great! Very easy to use and make delicious food providing you use correctly.
The people I've come across who complained about blandness etc were under seasoning and/or over watering their dishes. You don't need much liquid at all with them.
Fabulous for things like chilli, casseroles, soups and if you do want to go an Uber lazy route you can minimise prep but stay healthy by using pre-prepped ingredients - frozen, Tinned or fresh!
I used mine mostly when I was a single working mum and would just literally in the morning throw in
Protein - quorn mince, quorn chunks, soya or drained Tinned beans or pulses
Veg - frozen (was great for sneaking in frozen spinach quickly so dd didn't notice!) or drained Tinned veg, ready peeled and chopped veg like carrot batons or coins or diced butternut squash, broccoli, green beans etc
Carbs - ready peeled and diced white potato, sweet potato, dumplings
Flavouring - stock or stock cubes, herbs and spices (lazy garlic and ginger are staples here), or even use ready made slow cooker mixes
Though you could skip the carb at this stage and then when you're ready to eat cook some carbs quickly (cous cous only needs 5 mins, fresh pasta 3mins) or just add some bread on the side or even microwaved rice
Et voila dinner is served!
Was very handy when dd was at the "clubs every bloody night!" Stage and we didn't necessarily want to eat at the same times, as the person eating first could simply serve/be served and the dish kept warm for the later eater (usually me)